Absolute rate constants for one-electron oxidation of free base porphyrins (hematoporphyrin-IX, mesoprophyrin-IX dimethyl ester) by the trichloromethylperoxyl radical and by the radical cation of N-methylindole have been determined in a variety of solvents by the pulse radiolysis technique. All solvents were air-saturated and contained CCl4 as a source for the CCl3O2* radicals. The rate constants for reactions of these radicals were measured by following the formation of the porphyrin pi-radical cation optical absorption at 700 nm as a function of porphyrin concentration. The rate constants for reaction of CCl3O2* with the porphyrin were found to vary by an order of magnitude in the different solvents and were correlated with the solvent cohesive energy density. In the second set of experiments, N-methylindole was used in addition to CCl4 so that CCl3O2* radicals oxidize this solute to its radical cation, which in turn oxidizes the porphyrin. The variations of the rate constants for oxidation of the porphyrin by the N-methylindole radical cation in the different solvents exhibited different trends than those for oxidation by CCl3O2* radicals. For the reaction of N-methylindole radical cation with mesoporphyrin-IX dimethyl ester, it was possible to measure the oxidation potentials of the two compounds by cyclic voltammetry in seven solvents of widely varying polarities. The rate constants were then correlated with the free energy of the reaction and the solvent reorganization energy according to the Marcus equations.