cis-12-Oxophytodienoic acid (cis-12-oxo-PDA) is a C18 cyclopentenone formed from the 13-(S)-hydroperoxide of linolenic acid in flaxseed and other plant tissues. Although the structure of cis-12-oxo-PDA is well established, the absolute configuration of the side chains has not been determined. We have now measured this important parameter by two independent approaches. The CD spectrum of freshly prepared cis-12-oxo-PDA showed no deviations from base line--implying that the product is racemic. This conclusion was checked by a high-pressure liquid chromatography (HPLC) method capable of resolving the enantiomers; cis-12-oxo-PDA was reduced to two saturated hydroxy analogues which were each converted to (-)-menthoxycarbonyl diastereomers and analyzed by HPLC. Each epimer was resolved as two peaks of equal area, thus confirming that their cis-12-oxo-PDA parent is a racemic mixture, enantiomeric at the ring junctures. We propose that the biosynthesis of racemic cis-12-oxo-PDA proceeds by dehydration of the 13(S)-hydroperoxide to an allene oxide. A major fate of the allene oxide is hydrolysis to an alpha-ketol, which is always formed together with cis-12-oxo-PDA. The allene oxide also opens to a zwitterion, which undergoes charge delocalization to form a planar intermediate; this structure is the achiral precursor of the stable end product of pericyclic ring closure, viz., racemic cis-12-oxo-PDA.