Acid-catalyzed degradation of the methyl ester of 15(R)-15-methyl-prostaglandin F 2 α (2b) in acetonitrile afforded methyl 9α,11α-dihydroxy-15-methylprosta-5(Z),13,15-trienoate (3b) as the main product. Treatment of 2b with acidified methanol resulted in the formation of methyl 15(R,S)-9α,11α-dihydroxy-15-methoxy-15-methylprosta-5 (Z),13(E)-dienoate (4b,5b) as well as methyl 13(R,S)-9α,11α-dihydroxy-13-methoxy-15-methylprosta-5 (Z),14(E,Z)-dienoate (6b). In acidified aqueous 1,2-dimethoxyethane, 2b underwent epimerization into the corresponding 15(S) derivative (1b), solvolysis into methyl 13(R,S)-9α,11α,13-trihydroxy-15-methylprosta-5(Z),14(E,Z)-dienoate (9b), and dehydration into methyl 9α,11α-dihydroxy-15-methylprosta-5(Z),13,15-trienoate (3b). The same products, as the free acids, were formed from 15(S)-15-methyl-prostaglandin F 2 α (Carboprost, 1) upon treatment with acid in the solvents indicated. Degradation of the tromethamine salt of 15(S)-15-methyl-prostaglandin F 2 α (1c) in aqueous buffers was studied as a function of time, temperature, and pH. A gradual increase in stability of 1c was observed when the pH values of the buffers used were increased from 7.3 to 9.1 and higher. As a result of the stability tests, it was concluded that 1c may be stored at 37°C in tromethamine buffer, pH 9.55, for at least 1 year with less than 3-4% degradation.