Radiolytic reduction of chlororhodium(III) tetramesitylporphyrin ((ClRhP)-P-III) in alcohol solutions forms a transient (RhP)-P-II, which reacts to yield different products under different conditions. In alkaline 2-propanol the product is (RhP)-P-I-, in weakly acidic conditions (HRhP)-P-III is formed, and under strongly acidic conditions the main products of radiolysis are the alkylated rhodium complexes, R-(RhP)-P-III. The latter products are formed by reaction of (RhP)-P-II with alkyl radicals (R(.)) that are produced in the irradiated solvent (R(.) = (CH3)-C-. and (CH3)2(C)(.)OH in 2-propanol). UV photolysis of (ClRhP)-P-III in acetone/2-propanol solutions led to formation of HO(CH3)(2)C-(RhP)-P-III. One-electron reduction of CH3-(RhP)-P-III occurs at the porphyrin ligand to produce a transient pi-radical anion, CH3-(RhP.-)-P-III. In alkaline solution, this transient eliminates (.)CH3 to form the stable (RhP-)-P-I, but in neutral or acidic solutions, it undergoes disproportionation, promoted by protonation on the macrocycle, to form CH3-Rh-III-chlorin and then CH3-Rh-III-isobacteriochlorin upon further reduction. In the presence of CO2, the initial radiolytic reduction yields are increased. After extensive irradiation, however, the yields of reduction are decreased and a very low yield of CO was found. No reaction was detected between (RhP-)-P-I or (HRhP)-P-III and CO2, even under visible light illumination. Although this system is found to catalyze homogeneous photochemical formation of H-2, no catalytic activity for CO2 reduction was found under the current experimental conditions.