Rate constants for the homolysis of aryl-substituted α-cumyl tert-butyl peroxides (X- PhCMe2OOCMe3) produce a Hammett plot with a ρ value of− 0.22±. 04. Both steric and electronic parameters influence the decomposition rate constants of tertiary alkyl peroxides with bulky substituents: log krel=(− 0.40±08) Σσ*−(0.43±. 02) ΣEcs.