Cytochrome P-450 was demonstrated to catalyze the oxidative cleavage of carboxylic acid esters to the corresponding carboxylic acids. 2,6-Dimethyl-4-phenyl-3,5-pyridinedicarboxylic acid diethyl ester and related dialkyl esters were shown to serve as substrates in NADPH-fortified rat liver microsomes and reconstituted systems containing purified cytochrome P-450 enzymes. The ethyl group gave rise to acetaldehyde. The reactions proceed with large kinetic deuterium isotope effects, consistent with the view that P-450 abstracts a hydrogen atom in the mechanism. Oxygen rebound to the radical site is then postulated to complete the reaction and lead to a hemiacetal-like structure which collapses to give the products. Rate studies with differing alkyl substituents showed that the reaction was more rapid with removal of an ethyl than a methyl or isopropyl group, consistent with the view that the ethyl optimizes steric and inductive effects. Oxidative cleavage of carboxylic acid esters has little biochemical precedent, due to the difficult character of the reaction, and should be considered as an alternative to direct hydrolysis.