Peroxidation of mitochondria occurs extensively in ubiquinone-depleted membranes. Reincorporation into the membranes of either the physiological ubiquinone or a short-chain homologue protects mitochondria against peroxidation. The ability to prevent this phenomenon is more evident in mitochondria that have incorporated ubiquinone-3 and might be ascribed to an ordering structural effect on the lipid bilayer.