The decomposition of various organic peroxides has been studied as a primitive problem in chemistry and industry in order to clarify the characteristic properties of the peroxides.  Particularly, the structural change of tert-alkylperoxy moieties on the homolytic O-O bond scission and the reactivity of the free radical formed are examined by the decomposition of various tert-alkyl peroxides and hydroperoxides and clarified by the substituent effects of alkyl group and the assumption of the decomposition mechanism.  Consequently, it is suggested that the decomposition reaction proceeds via the polar activation complex which is stabilized by the electron-donating alkyl substituents and stretches slightly the C –C bond neighboring to the peroxy oxygen atom.