The nonenzymatic isoprostane pathway of lipid peroxidation of polyunsaturated fatty acids results in formation of products, termed isoprostanes, which have very large positional and stereo isomerism, possess various biological activities, produce adducts with proteins, and thus contribute to pathogeneses of the age-dependent diseases. However, it was unclear what mechanism drives this type of lipid autoxidation, and why the products have very large isomerism. We propose a mechanism when perhydroxyl radicals (HO₂^•) react with polyunsaturated fatty acids in the hydrophobic milieu of membranes. In the membrane HO₂^• initiates a chain of reactions with formation first H₂O₂, which undergoes homolytic fission producing two ^•OH radicals, thus very rapidly abstracting three H atoms from a polyunsaturated fatty acid. As a result, the HO₂^• molecule is converted to two molecules of water, and the molecule of a polyunsaturated fatty acid loses two double bonds, becomes highly unstable and undergoes peroxidation and random intramolecular re-arrangements causing a very large isomerism of the final products. The extremely high reactivity of ^•HО₂ with polyunsaturated fatty acids is the cause of very subtle and slow accumulation of damages in the membrane and membrane associated proteins, even though the concentration of ^•HО₂ relative to superoxide radical may be very low.