Certain thiol-containing compounds catalyze, in a chemical reaction, the isomerization of all-trans-retinoic acid (RA) to 13-cis-RA and of 13-cis-RA to RA. Reactions approaching equilibrium contain more RA than 13-cis-RA. Small molecules effective as catalysts are glutathione, mercaptoethanol, and L-cysteine methyl ester. L-Cysteine is not a catalyst and inhibits the reaction catalyzed by glutathione or mercaptoethanol. Apoferritin (a thiol-containing protein), native microsomes, and, to a lesser extent, boiled microsomes catalyze the reaction, but their activity is reduced or eliminated by prior incubation with iodoacetate. Other cis and trans isomeric retinoids are also substrates for this reaction; the reactions proceed more readily for the cis isomers. The thiol-catalyzed isomerization of RA and 13-cis-RA may account for the observations of both cis and trans forms of retinoids in tissues of animals after administration of either.