Phospholipids containing trans-unsaturated fatty acid residues are the major products of the thiyl radical attack on L-alpha -phosphatidylcholine from soybean lecithin in homogeneous solution or in liposomes (LUVET). Thiyl radicals act as the catalyst for the cis-trans isomerization, and the number of catalytic cycles depends on the reaction conditions. The presence of similar to0.2 mM oxygen does not influence the reaction outcome but accelerates the efficiency of cis-trans isomerization in homogeneous solution. Under these conditions, the PUFA peroxidation is found to be unimportant. A detailed study of the isomerization of methyl linoleate including product studies indicates the formation of a small amount of conjugated dienes that act as inhibitors. Indeed, all-trans-retinol substantially retarded the isomerization process.