1. 1. The degradation of linoleic acid hydroperoxide by cysteine and FeCl 3 resulted in formation of a number of oxygenated fatty acids, among which isomeric epoxyoxooctadecenoic and epoxyhydroxyoctadecenoic acids were major products. Pure isomeric hydroperoxides, either 13- l(S)- hydroperoxy-cis-9 , trans-11-octadecadienoic acid or 9- d(S)- hydroperoxy-trans-10 , cis-12-octadecadienoic acid, were transformed into either 12,13-epoxides or 9,10-epoxides, respectively. 2. 2. From 13- l(S)- hydroperoxy-cis-9 , trans-11-octadecadienoic acid, the epoxides were identified as trans-12,13-epoxy-9-oxo- trans-10-octadecenoic acid, trans-12,13-epoxy-9-hydroxy- trans-10-octadecenoic acid, cis-12,13-epoxy-9-oxo- trans-10-octadecenoic acid, trans-12,13-epoxy- erythro-11-hydroxy- cis( trans)-9-octadecenoic acid and trans-12,13-epoxy- threo-11-hydroxy- cis( trans)-9-octadecenoic acid. 3. 3. The 12,13-epoxides were found to be optically active, indicating that the chiral center of the 13- l(S)- hydroperoxy carbon was retained. 4. 4. Although many epoxy fatty acids previously have been identified as linoleic acid hydroperoxide products, this study reports a more complete structural analysis of the various epoxides and allows an assessment of the mechanisms of their formation from hydroperoxides.