The radical decompositions of eight kinds of alkyl α, α′-azobisiso-butyrates in various solvents, such as cyclohexane, benzene, acetoni-trile, methanol, and acetic acid, and in methanol solution of stannic chloride or zinc chloride were investigated. It was found that the methyl α, α′-azobisisobutyrate decomposed through first-order kinetics in these solvents, but the resulting rate constants changed significantly with the kind of solvent or metal chloride used. These rate constants were also observed to correlate with the polarities and the proton-donating abilities (ETvalues) of the solvents used. The activation enthalpies and entropies at 60°C were determined; the former values were kept constant independent of the reaction medium, but the latter values were changed.