AbstractRecently, corn (Zea mays L.) hydroperoxide dehydrase was found to catalyze the conversion of 13(S)‐hydroperoxy‐9(Z),11(E)‐octadecadienoic acid into an unstable fatty acid allene oxide, 12,13(S)‐epoxy‐9(Z),11‐octadecadienoic acid. This study is concerned with the chemistry of 12,13(S)‐epoxy‐9(Z),11‐octadecadienoic acid in the presence of vertebrate serum albumins.Albumins were found to greatly enhance the aqueous half‐life of the allene oxide, i.e. 14.1±1.8 min, 11.6±1.2 min and 4.8±0.5 min at 0 C in the presence of 15 mg/ml of bovine, human and equine serum albumins, respectively, as compared with ca. 33 sec in the absence of albumin. Degradation of allene oxide in the presence of bovine serum albumin led to the formation of a novel cyclization product, i.e. 3‐oxo‐2‐pentyl‐cyclopent‐4‐en‐1‐octanoic acid (12‐oxo‐10‐phytoenoic acid, in which the relative configuration of the side chains attached to the five‐membered ring istrans). Steric analysis of the cyclic derivative showed that the compound was largely racemic (ratio between enantiomers, 58∶42).12‐Oxo‐10,15(Z)‐phytodienoic acid, needed for reference purposes, was prepared by incubation of 13(S)‐hydroperoxy‐9(Z),11(E),15(Z)‐octadecatrienoic acid with corn hydroperoxide dehydrase. Steric analysis showed that the 12‐oxo‐10,15(Z)‐phytodienoic acid thus obtained was not optically pure but a mixture of enantiomers in a ratio of 82∶18.