Fe (II)-mediated decomposition of 13-[S-(Z,E)]-9,11-hydroperoxyoctadecadienoic (hydroperoxy-linoleic) acid resulted in the formation of three α,β-unsaturated aldehydes. At low Fe (II) concentrations or at early time points after the addition of Fe(II), two major products were observed. The least polar product had chromatographic properties that were identical with those of 4-oxo-2-nonenal. Conversion of this product to its bis-oxime derivative with hydroxylamine hydrochloride resulted in two syn- and two anti-oxime isomers that had chromatographic and mass spectral properties identical with the properties of products derived from an authentic standard of 4-oxo-2-nonenal. This confirmed for the first time that 4-oxo-2-nonenal is a major product of the Fe(II)-mediated breakdown of lipid hydroperoxides. The more polar product had chromatographic properties that were similar to those of 4-hydroperoxy-2-nonenal. LC/MS analysis of its syn- and anti-oxime isomers confirmed this structural assignment. Thus, 4-hydroperoxy-2-nonenal is a previously unrecognized major product of lipid hydroperoxide decomposition. At high Fen concentrations and at longer incubation times, a third more polar product was observed with chromatographic properties that were identical to those of 4-hydroxy-2-nonenal. The syn- and anti-oxime isomers had chromatographic and mass spectral properties identical with the properties of products derived from an authentic standard of 4-hydroxy-2-nonenal. It appears that 4-hydroperoxy-2-nonenal is formed initially and that it is then converted to 4-hydroxy-2-nonenal in the presence of high Fe (II) concentrations or by extended incubations in the presence of low Fen concentrations. It is conceivable that some of the 4-hydroperoxy-2-nonenal is also converted to 4-oxo-2-nonenal. However, we cannot rule out the possibility that it is also formed by a concerted mechanism from a rearrangement product of 13-[S-(Z,E)]-9,11-hydroperoxyoctadecadienoic acid.