Abstract The first-order rate constants ( k i ) and efficiencies of radical formation (ε) for azo- p-cymene were measured in ten solvents spanning 80-fold in absolute viscosity and 30-fold in dielectric constant. Although viscosity plays a major role in the cage effect, specific solvation of p-cymyl radicals must also be considered. The k i exhibit little change with solvent viscosity or dielectric constant, and no trend is apparent. Activation parameters were measured in the most polar solvent (propylene carbonate) and one of the least polar solvents (n-heptadecane). The results indicate either a lack of solvent participation in the thermolysis, or similar solvation of the ground and transition states in both solvents.