It has been found that the reaction of the trans-(2a or 3a) or the cis-isomer (2b or 3b) of 5-ethyl-2-oxo-4-piperidineacetic acid (2) or its ethyl ester (3) with 10% EtOH-HCl at reflux for 20-27 hr gave an equilibrated 70 : 30 mixture of the trans-(3a) and the cis-isomer (3b) of the ester (3) in a good yield. Such a cis-trans isomerization did not occur at all when the trans-(2a) or the cis-lactam acid (2b) was similarly esterified but at 15°for 16 hr, whereas at 32°it did occur to a slight extent. In contrast to the N-unsubstituted lactam acids (2a, b), trans-(5a) and cis-1-benzyl-5-ethyl-2-oxo-4-piperidineacetic acid (5b) could be converted into the corresponding ethyl esters (7a, b) by similar esterifications even at 32°with complete retention of their original stereochemistry. The cis-trans isomerization of the methyl esters (6a, b) of 2a, b in 10% MeOH-HCl seemed somewhat easier than that of the ethyl esters (3a, b) in 10% EtOH-HCl. On the other hand, the N-benzylated methyl esters (8a, b) did not isomerize in 10% MeOH-HCl even after 5 hours'reflux.