The absolute rates of reaction of the trichloromethylperoxy radical, CCl3OO., derived from carbon tetrachloride and the halothane peroxy radical, CF3CHClOO., with oleic, linoleic, linolenic and arachidonic acids have been determined using the fast reaction technique of pulse radiolysis. In general, the rates of reaction of the radical derived from carbon tetrachloride are approximately five times greater than those for the halothane related radical. In both cases the rate constant increases with increasing unsaturation of the fatty acid in agreement with the known greater susceptibility of polyunsaturated fatty acids to peroxidative decomposition.