The thermal oxidation of the membranes of linoleic acid vesicles was preceded by a lag period, as long as the membranes contained low levels of preformed peroxides. Incorporation of 0.034 to 0.170 mol% of nitroxide spin label increased the length of this lag between 4.8 and 10.1 times. At the same time, the intensity of the ESR signal fell. The inclusion of as little as 0.04 mol% of butylated hydroxytoluene in the membranes also lengthened the lag period by a factor of 2.5. However, a similar molar proportion of α-tocopherol was without effect. When the linoleic acid from which vesicle membranes were formed contained between 0.45 and 1.43 mol% of peroxide, α-tocopherol produced a significant increase in the lag period, during which the antioxidant was gradually oxidized.