Epoxides of fatty acids are hydrolyzed by epoxide hydrolases (EHs) into dihydroxy fatty acids which are of particular interest in the mammalian leukotriene pathway. In the present report, the analysis of the configuration of dihydroxy fatty acids via their respective hydroxylactones is described. In addition, the biotransformation of (AE)-erythro-7,8-and -3,4-dihydroxy fatty acids in the yeast Saccharomyces cerevisiae was characterized by GC/EI-MS analysis. Biotransformation of chemically synthesized (AE)-erythro-7,8dihydroxy(7,8-2 H 2 )tetradecanoic acid ((AE)-erythro-1) in the yeast S. cerevisiae resulted in the formation of 5,6-dihydroxy(5,6-2 H 2 )dodecanoic acid (6), which was lactonized into (5S,6R)-6-hydroxy(5,6-2 H 2 )dodecano-5-lactone ((5S,6R)-4) with 86% ee and into erythro-5-hydroxy(5,6-2 H 2 )dodecano-6-lactone (erythro-8). Additionally, the a-ketols 7-hydroxy-8-oxo(7-2 H 1 )tetradecanoic acid (9a) and 8-hydroxy-7oxo(8-2 H 1 )tetradecanoic acid (9b) were detected as intermediates. Further metabolism of 6 led to 3,4dihydroxy(3,4-2 H 2 )decanoic acid (2) which was lactonized into 3-hydroxy(3,4-2 H 2 )decano-4-lactone ( 5) with (3R,4S)-5 ¼ 88% ee. Chemical synthesis and incubation of (AE)-erythro-3,4-dihydroxy(3,4-2 H 2 )decanoic acid ((AE)-erythro-2) in yeast led to (3S,4R)-5 with 10% ee. No decano-4-lactone was formed from the precursors 1 or 2 by yeast. The enantiomers (3S,4R)-and (3R,4S)-3,4-dihydroxy(3-2 H 1 )nonanoic acid ((3S,4R)-and (3R,4S)-3) were chemically synthesized and comparably degraded by yeast without formation of nonano-4-lactone. The major products of the transformation of (3S,4R)-and (3R,4S)-3 were (3S,4R)-and (3R,4S)-3-hydroxy(3-2 H 1 )nonano-4-lactones ((3S,4R)-and (3R,4S)-7), respectively. The enantiomers of the hydroxylactones 4, 5, and 7 were chemically synthesized and their GC-elution sequence on Lipodex E chiral phase was determined.