The ability of glutathione to catalyze the cis-trans isomerization of acitretin and isoacitretin was explored. Glutathione catalyzed the isomerization reaction in both directions; the reaction rate varied with glutathione concentration, atmospheric exposure condition, and identity of starting isomer. The ability of this widely distributed molecule to catalyze this interconversion nonenzymatically may contribute to the presence of both isomers in vivo after administration of either isomer.