Thiyl radical-mediated transformation of unsaturated fatty acid residues are reported. Beside the initiation of lipid peroxidation, thiyl radicals can efficiently cause isomerization of fatty acid residues in a catalytic manner. The latter process is observed in homogeneous solutions as well as in organized assemblies, leading to a denaturation of the natural all-cis-isomers of fatty acid residues of lipid bilayers. The degree of denaturation goes parallel with an increase in the number of double bonds and in the hydrophobicity of thiols. For arachidonic acid methyl ester, a decay of the all-cis-isomer to approximately 30% after a gamma irradiation dose of 1.8 kGy in micelles is observed. The reversibility of the isomerization reaction is demonstrated in liposomes by the fact that the cis/trans-equilibrium is independent of configuration of the starting compound.