Ethyl esters and phosphatidylcholines (PCs) of polyunsaturated fatty acids (PUFAs) were oxidized in organic solvents, aqueous emulsions, and liposomes in the presence of a radical inducer. Oxidation products and the positional distribution of monohydroperoxide (MHP) were determined by gas chromatography-mass spectrometry (GC-MS) analysis. The total amount of the oxidation products, of PUFA ethyl esters and PCs in organic solvents, increased with an increase in the number of bis-allylic positions. However, the opposite results were obtained in an aqueous emulsion and liposomes. The distribution pattern of MHPs obtained from oxidation of the linolate and alpha-linolenate showed little difference between a chloroform solution and an aqueous emulsion or liposomes. However, there were differences between these systems with the arachidonate, the icosapentaenoate, and docosahexaenoate. These results may be due to the different rate of hydrogen abstraction from bis-allylic positions in the fatty acid moieties, and/or 1,3-cyclization of hydroperoxides in the systems. (C) 2003 Elsevier Ireland Ltd. All rights reserved.