The stereochemistry of the four stereoforms of 5-chloro-3-methylmuconolactones could be deduced from NMR and stability data, and from the comparison with authentic (4R, SS)-5-chloromuconolactone. Muconolactone isomerase of Alcaligenes eutrophus JMP 134 was shown to catalyze syn-elimination of hydrogen chloride from (4R, 5R)-S-chloro-3-methylmuconolactone, (4R, SS)-S-chloro-3-methylmuconolactone and (4R, SS)-5-~hloromuconolactone to form 3-methyl-trans-dienelactone, 3-methyl-cisdienelactone and a 3 : 1 mixture of cis-and trans-dienelactone, respectively. 3-Methyl-trans-dienelactone was a substrate of pJP4-encoded dienelactone hydrolase of A. eutrophus JMP 134, whereas 3-methyl-cisdienelactone transformation was negligible indicating a restricted substrate specificity of this enzyme.Both substrates were transformed into 3-methylmaleylacetate which in turn was a substrate for maleylacetate reductase. This compound was shown to possess a cyclic structure (4-hydroxy-3-methylmuconolactone) under acidic conditions.