Abtract The ratios of 13:9 isomers of the hydroperoxides produced during incubation of methyl linoleate with soybean lipoxygenase-1 were ca. 50:50 at pH 7.8 and ca. 70:30 at pHs 9.0 and 11.0, respectively. In striking contrast to the lipoxygenase-catalyzed hydroperoxidation of linoleic acid, the hydroperoxidation of methyl linoleate at pH 7.8 gave almost racemic 13-hydroperoxides and that at pHs 9.0 and 11.0 yielded methyl 13-L-hydroperoxylinoleate having low optical purities (L/D, ca. 45/30).