What Is ACS Referencing?

If you’re editing or proofreading academic writing about chemistry, you may need to know how the ACS referencing system works. But you might not have come across this citation style before unless you’re a chemist! That’s why we’ve put together this guide to take you through the essentials.

What Is ACS Referencing?

The “ACS” in “ACS referencing” stands for the American Chemical Society, who developed the system as part of The ACS Style Guide. As with any system for references and citations, it aims to give writers a clear, consistent approach to citing sources in academic work.

You can find the most recent version of this system in The ACS Guide to Scholarly Communication, which is available online for a subscription.

The tricky thing about ACS, though, is that it offers two distinct citation styles:

• An author–date system – This method requires writers to include the author’s name and the date of publication for sources in parentheses within the text, with full source information in a reference list.
• A number–note system – In this, writers cite sources in the text with numbers that point to an entry in a reference list at the end of the document.

We’ll look at both approaches in more detail below.

Author–Date Citations

The author–date version of ACS style, like other parenthetical referencing systems, requires writers to give the author’s name and the date the work was published in parentheses when citing sources:

The explosion caused severe damage to the laboratory (Harris, 2020).

It’s important to always check the correct use of punctuation when proofreading. So note that when the citation falls at the end of the sentence, it comes before the closing period.

If the author is already named in the text, though, only the year is included in the parentheses:

Harris (2020) reports that the explosion caused severe damage to the laboratory.

If a work was written by two authors, which is often the case in scientific research articles, the names are connected with “and” whether they are part of the sentence or included in parentheses:

Jekyll and Hyde (2023) identified the neurological basis of identity fragmentation.

Identification of the neurological basis of identity fragmentation was a lengthy process (Jekyll and Hyde, 2023).

And for sources with more than three authors, ACS allows the use of “et al.” (a Latin term meaning “and others”) after the first author’s name to save space:

Other experiments have been more controlled (Kaos et al., 2004).

Kaos et al. (2004) noted how other experiments have been more controlled.

Even if “et al.” is used in the citation, all authors should still be named in the entry in the reference list at the end of the text.

Number–Note Citations

The ACS allows two different styles of number–note referencing. 

One version uses superscript numbers to indicate citations:

The explosion caused severe damage to the laboratory.¹

And the other uses italicized numbers in parentheses:

The explosion caused severe damage to the laboratory (1).

Pay close attention to the punctuation in the above examples. When the citation applies to the whole sentence, the number in parentheses comes before the closing punctuation (just like in author–date style). But a superscript number comes after the period.

No matter which style is used, number–note citations follow certain rules:

• Sources are numbered sources sequentially (i.e., the first source a writer cites in their work will be “1,” the second source will be “2,” the third “3,” and so on).
• That same number is used each time the same source is cited.

Each number directs the reader to an entry in the reference list, which provides full information for the source material.

Individual writers, journals, and institutions will favor one approach over the other. Therefore, you’ll need to check which version applies to the document at hand if you’re asked to use ACS referencing.

Naming the Author with Number–Note Citations

A writer can attach a citation to a statement or fact, as shown in the examples above, or they can include it after mentioning the name of the author. If this is the case, the number should come immediately after the name. For instance:

Superscript: Herman¹ claims the results show a significant improvement.

Parenthetical: Herman (1) claims the results show a significant improvement.

In the same way as with author–date citations, if a source has two authors, the names should be joined with “and” in both styles of number–note citation:

Superscript: Smith and Jones² achieved similar results in a later test.

Parenthetical: Smith and Jones (2) achieved similar results in a later test.

And as we mentioned above, a list of three or more names can be cut down to to the first name followed by “et al.” to keep things concise:

Superscript: Harris et al.³ were initially skeptical.

Parenthetical: Harris et al. (3) were initially skeptical.

Note how the citation is followed by a plural verb (“were”). Even though there is only a single surname (“Harris”), the phrase as a whole means “Harris and others,” so it needs to be followed by a plural verb to ensure subject–verb agreement.

Citing Multiple Sources at Once

ACS referencing also allows writers to cite more than one source at once. This isn’t uncommon in scientific writing, where one author may have written several works or where an explanation may relate to multiple different sources.

Let’s take a look at how this works in each style.

Multiple Citations in Author–Date Style

If an author published several works that deal with the same topic, a writer might want to include multiple citations by the same person to support one statement. This is what that would look like:

How shapes seen in clouds reflect the viewer’s state of mind has been studied in depth (Cirrus, 1998, 2003).

And if the writer found information on a topic in works by multiple authors, they may want to include citations for each one:

Seeing clouds shaped as bunnies could imply a playful state of mind (Cirrus, 2003; Lepus, 2007).

Multiple Citations in Number–Note Style

When it comes to including multiple citations in numerical style, we have even more options available! 

If the writer is citing two sources at once, the numbers are separated with a comma. However, there is a slight difference depending on if the numbers are superscript or in parentheses. See if you can spot it:

Superscript: Much of the universe is made up of the space between atoms.^2,6

Parenthetical: Much of the universe is made up of the space between atoms (2, 6).

The superscript numbers are separated by a comma with no space, while the parenthetical numbers take a space as well as the comma.

If there are more than two items in the citation, number–note style is all about efficiency! The basis is that if the sources are in order in the reference list, they don’t need to be provided individually in the citation. Instead, the writer can use an en dash to show the range of numbers:

Superscript: The results caused significant debate over the following year.^8–11

Parenthetical: The results caused significant debate over the following year (8–11).

In the examples above, then, the writer is citing sources 8, 9, 10, and 11 from the reference list.

And if there is a mix of consecutive and nonconsecutive sources, the citation can include both commas and dashes between the numbers:

Superscript: This theory has been tested repeatedly without success.^1,4–8

Parenthetical: This theory has been tested repeatedly without success (1, 4–8).

Finally, to cite more than one source by the same principal author working with different co-authors, we can give the first author’s name followed by “and coworkers” or “and colleagues” and then the relevant numbers:

Superscript: Harris and colleagues^1,10–14 have run multiple tests to confirm this.

Parenthetical: Smith and coworkers (19–22) produced a lot of evidence.

As you can see, there’s a lot to keep an eye out for when proofreading works about chemistry!

ACS Reference Lists

ACS reference lists should always contain full publication information for all sources cited, regardless of the specific format used. But there is a difference in how they’re organized:

• The number–note system lists sources in the order they’re first cited. This is so the position in the reference list matches the citation numbers used in the text.
• The author–date version lists sources alphabetically by author surname. This makes it easy for readers to find the full information from the names mentioned in the in-text citations.

The exact format for a reference list entry will depend on the source type (e.g., a book, journal article, or website). However, the ACS does provide some general principles for references:

• Books – References should include the author or editor names, the book title, the publisher, the city of publication, and the year of publication.
• Journal articles – List the author names, the abbreviated journal title, the year of publication, the volume number, and the complete page range of the article.
• Other sources – Provide enough detail for the source to be identified and located.

We’ll admit that this last one is unhelpfully vague, but you can find examples of various source types in The ACS Style Guide.

Becoming A Proofreader

We hope you found this guide to ACS referencing helpful! It’s not the most common citation style, but it is still vital to get the details right in any kind of academic writing.

If you want to learn more, our Becoming A Proofreader course contains whole modules on academic writing and different referencing styles! With our comprehensive training and accessible tutor support, you could be well on your way to proofreading professionally in no time. Take the first step by trying two free lessons today.

Updated on March 31, 2025