Hydrocarbon gases have been used previously as an index of lipid peroxidation in vivo and in vitro. In vitro experiments are reported on the formation of hydrocarbon gases from peroxidizing omega-3 and omega-6 fatty acids. Hydrocarbon gases were not released during a 20-hr peroxidation phase but were released following the decomposition of hydroperoxides by addition of excess ascorbic acid. The major hydrocarbon gas products in iron, copper, or hematin catalyzed peroxidation systems were ethane or ethylene from linolenic acid, and pentane from linoleic acid and arachidonic acid. Calculations of the ratios of hydrocarbon gases formed were based on fatty acid decrease and/or change in diene conjugation and peroxide values. Depending on the fatty acid, catalyst, and calculation basis used, pentane formation was as high as 1.3 mol %, ethane 4.3 mol %, and ethylene 10.6 mol %.