Karrers early research concerned complex metal compounds but his most important work has concerned plant pigments, particularly the yellow carotenoids. He elucidated their chemical structure and showed that some of these substances are transformed in the body into vitamin A. His work led to the establishment of the correct constitutional formula for beta-carotene, the chief precursor of vitamin A; the first time that the structure of a vitamin or provitamin had been established. George Wald worked briefly in Karrers lab while studying the role of vitamin A in the retina. Later, Karrer confirmed the structure of ascorbic acid (vitamin C) and extended his researches into the vitamin B and E. His important contributions to the chemistry of the flavins led to identification of lactoflavin as part of the complex originally thought to be vitamin B.
Karrer published many papers, and received many honours and awards, including the Nobel Prize in 1937. His textbook Lehrbuch der Organischen Chemie (Textbook of Organic Chemistry) was published in 1927, went through thirteen editions, and was published in seven languages.