Frances H. Arnold awarded the 2025 ACS Priestley Medal

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Frances Arnold, winner of the 2025 Priestley Medal.
Christopher Michel for Caltech

WASHINGTON, July 8, 2024 — The American Chemical Society (ACS) is proud to announce that Frances H. Arnold is being awarded the 2025 Priestley Medal. This award is the highest honor bestowed by ACS, and it annually recognizes an individual for distinguished service to chemistry. Arnold is being honored "for her pioneering contributions to the development of directed evolution as a method for chemical and biological design." She was also awarded the Nobel Prize in chemistry in 2018 for the technique.

This methodology has enabled significant advancements in industrial biocatalysis, pharmaceuticals and the development of sustainable chemical processes. Arnold has continued to develop it since the early 1990s, and it has changed how scientists improve and use enzymes in various industries. By making small changes and selecting the best results, Arnold’s method has allowed the creation of better enzymes for use in medicine and environmental solutions.

The technique is used in industrial laboratories to optimize enzymes, antibodies and other therapeutic proteins; to alter metabolic pathways in microorganisms to enhance their ability to convert raw materials to higher-value biological products; and even to modify whole organisms. Arnold showed that enzymes could be “bred” for behavior that may not be relevant in a natural context but is important for human use.

“She is a distinguished scientist, a pioneering engineer, a wonderful role model for young men and women, and a successful entrepreneur who has had extraordinary impact on the way we think about chemical and biological design and the biotechnology industry,” says David Tirrell, provost, the Ross McCollum-William H. Corcoran Professor of Chemistry and Chemical Engineering, and the Carl and Shirley Larson Provostial Chair at the California Institute of Technology (Caltech).

Arnold holds several positions, including co-chair of the President’s Council of Advisors on Science and Technology and Linus Pauling Professor of Chemical Engineering, Bioengineering, and Biochemistry at Caltech. She is also the director of the Donna and Benjamin M. Rosen Bioengineering Center. She earned her undergraduate degree in mechanical and aerospace engineering from Princeton University and a Ph.D. in chemical engineering from the University of California, Berkeley.

Arnold will formally accept the Priestley Medal and deliver an address at ACS Spring 2025 in San Diego.

Nominations for the 2026 Priestley Medal are open through Nov. 1, 2024. To nominate someone for an ACS national award, visit the awards page.


The American Chemical Society (ACS) is a nonprofit organization chartered by the U.S. Congress. ACS’ mission is to advance the broader chemistry enterprise and its practitioners for the benefit of Earth and all its people. The Society is a global leader in promoting excellence in science education and providing access to chemistry-related information and research through its multiple research solutions, peer-reviewed journals, scientific conferences, eBooks and weekly news periodical Chemical & Engineering News. ACS journals are among the most cited, most trusted and most read within the scientific literature; however, ACS itself does not conduct chemical research. As a leader in scientific information solutions, its CAS division partners with global innovators to accelerate breakthroughs by curating, connecting and analyzing the world’s scientific knowledge. ACS’ main offices are in Washington, D.C., and Columbus, Ohio.

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