Enzyme-catalyzed esterifications of thio and carboxylic acid analogues and thiotransesterifications are studied using a commercial immobilized Mucor miehei lipase preparation in an organic solvent. For identical conditions, thio acids are found to react with butanol at rates much faster than those for the corresponding carboxylic acids, suggesting that these substrates could be used to increase the rate of lipase-catalyzed ester syntheses. Enzymatic reactions of butanol and (S)-3-(acetylthio)-2-methylpropionic acid were found to occur via a complex mechanism leading to the formation of multiple products. This reaction appears to occur in a sequential manner, following a parallel-consecutive mechanism yielding initially the carboxylic acid butyl ester and later the deacetylated thiomethylpropionic acid butyl ester.