Chaotropic agents induce lipid oxidation in submitochondrial particles and microsomes by destabilizing the native structure of these membranes. A plot of the rate of lipid oxidation as a function of chaotrope concentration shows a cooperativity effect similar to that observed in the unfolding of a protein molecule by a perturbing agent. However, the magnitudes of the activation parameters (E a, ΔH‡, ΔF‡,and ΔS‡) suggest that at high chaotrope concentration structure destabilization has little influence on the rate of lipid oxidation. Erythrocyte ghosts and Bacillus subtilis plasma membranes, which have a low content of acid-labile sulfide and cytochromes, resist lipid autoxidation. Kinetic data of lipid oxidation and labile sulfide loss in the presence of chaotropic agents are consistent with the possibility that iron-sulfur proteins, or their intermediate decomposition products, catalyze the initial, rapid phase of autoxidation in submitochondrial particles. Results are discussed in relation to the structure of membranes and the structure of water.