The kinetics of decomposition of 3,5,5-trimethylhexanoyl peroxide(1)in benzene at 30,40,and 50℃ respectively have been studied and the cage effect,main products and the productsobtained in scavenging experiment with galvinoxyl determined and characterized.The results showthat the decomposition of 1,with initial concentration between 0.04 to 0.43 mol.L~(-1),followed firstplus three halves order kinetics with cage effect of 0.6,and the mechanism of decomposition is basi-cally the same as previously proposed for lauroyl peroxide(2)~4 In comparison with 2,the decom-position of 1 at the same temperature showed a faster rate for spontaneous decomposition,a largercage effect and lesser induced decomposition,which is attributed to the branching in molecule of 1,especially to the presence of β-methyl group,which caused a larger entropy increase between theperoxide molecule and the transition state for decomposition. The kinetics of decomposition of 3,5,5-trimethylhexanoyl peroxide (1) in benzene at 30,40, and 50 ℃ respectively have been studied and the cage effect, main products and the product so scotching experiment with galvinoxyl determined and characterized. results showthat the decomposition of 1, with initial concentration between 0.04 to 0.43 mol·L -1, followed by firstplus three halves order kinetics with cage effect of 0.6, and the mechanism of decomposition is basi-cally the same as previously proposed for lauryl peroxide (2) ~ 4 In 2, the decom-position of 1 at the same temperature showed a faster rate for spontaneous decomposition, a larger effect and lesser induced decomposition, which is attributed to the branching in molecule of 1, especially to the presence of β-methyl group, which caused a larger entropy increase between the peroxide molecule and the transition state for decomposition.