How to Format an In Text Citation American Chemical Society

The basic format for an American Chemical Society (ACS) in text citation uses the author’s last name and the year of publication, placed in parentheses. For example: (Smith, 2020). This author-date system is common in ACS style, though numbered citation systems are also accepted depending on the publication. The key is to provide enough information for the reader to find the full reference in your bibliography without interrupting the flow of your sentence.

Understanding the Author-Date System

The author-date system is the default for many ACS publications. You place the citation at the end of the relevant sentence or clause, inside the punctuation.

Basic format: (Author Last Name, Year)

Single author: (Jones, 2021)

Two authors: (Jones and Smith, 2021) — note the use of “and,” not an ampersand.

Three or more authors: (Jones et al., 2021) — use “et al.” in italics or plain text, but be consistent. The ACS Style Guide allows both, but plain text is simpler for most writers.

Placement in a sentence

You can also integrate the author name into the sentence itself, with the year in parentheses.

• Jones (2021) reported that the reaction rate increased.
• In a study by Jones and Smith (2021), the catalyst showed improved stability.

When the author is unknown

If the source has no identifiable author, use a shortened version of the title in quotation marks. For example: (“Analysis of Polymer Stability,” 2020). If the title is long, shorten it to the first few key words.

Multiple sources in one citation

List them in alphabetical order by author, separated by semicolons. For example: (Brown, 2019; Jones, 2021; Smith, 2020).

Using the Numbered Citation System

Some ACS journals and many textbooks prefer a numbered system. In this method, you assign a number to each source as you first cite it. The number appears in the text, usually as a superscript or in brackets.

Superscript format: The reaction was complete after 2 hours.^1

Bracketed format: The reaction was complete after 2 hours [1].

Multiple sources: …as shown in previous studies [1,2,4-6].

The numbered system is efficient for papers with many citations because it avoids cluttering the text with author names. The full reference list then appears in numerical order at the end of the document.

Choosing between author-date and numbered

If you are writing for a specific journal, check its author guidelines. For a student paper, ask your instructor. The author-date system is generally clearer for readers who want to identify the source quickly. The numbered system is cleaner for dense scientific writing.

Handling Special Cases

Certain source types require slight adjustments to the standard format.

Corporate or group authors

If the author is an organization, spell out the full name in the first citation. You can use an abbreviation in subsequent citations if it is clear.

• First citation: (American Chemical Society [ACS], 2020)
• Subsequent citations: (ACS, 2020)

Direct quotations

If you quote a source directly, include the page number. For example: (Smith, 2020, p. 45). For online sources without page numbers, use a paragraph number or section heading if available.

Secondary sources

If you are citing a source that you found quoted in another source, use the phrase “as cited in.” For example: (Brown, 2018, as cited in Jones, 2021). You should try to find the original source whenever possible.

Electronic sources

For websites, reports, or online articles, use the same author-date format. If there is no publication date, use “n.d.” (no date). For example: (American Chemical Society, n.d.). The ACS Style Guide notes that you should include a retrieval date only if the content is likely to change over time, such as a wiki page.

Formatting the Reference List

Every in text citation must correspond to a full reference in your bibliography or reference list. The reference list provides complete publication details so the reader can locate the source.

Book format: Author Last Name, First Initial. Book Title; Publisher: Location, Year.

• Jones, A. Principles of Organic Chemistry; Oxford University Press: New York, 2020.

Journal article format: Author Last Name, First Initial. Article Title. Journal Abbreviation Year, Volume, Page Range.

• Smith, B. Synthesis of Novel Polymers. J. Am. Chem. Soc. 2021, 143, 1234-1240.

Website format: Author Last Name, First Initial. Title of Page. Publisher, Publication Date. URL (accessed Date).

• American Chemical Society. ACS Style Guide. https://www.acs.org (accessed March 15, 2025).

The reference list should be alphabetized by author last name for the author-date system. For the numbered system, references appear in the order they are first cited in the text.

A quick example

In your paper, you might write: “The catalyst developed by Chen (2022) showed a 30% increase in yield.” Then in your reference list, you would include:

• Chen, L. Catalysis for Green Chemistry; Springer: Berlin, 2022.

This pairing ensures that every citation is traceable.

Key Takeaways

• The standard ACS in text citation format is author-date: (Author, Year).
• For three or more authors, use “et al.” after the first author.
• The numbered citation system uses superscript or bracketed numbers and is also accepted.
• Always include a full reference list that matches your citation style.
• For direct quotes, add page numbers; for corporate authors, spell out the name on first use.