Rhodium(III) tetramesitylporphyrin (Rh(III)TMP) undergoes radiolytic reduction in deoxygenated alkaline alcohol solutions to produce a transient Rh(II)TMP and then a stable Rh(I)TMP. When the solvent was 2-PrOH and the KOH concentration was >0.02 mol L(-1), the radiolytic yield of reduction was considerably higher than the total yield of reducing radicals, indicating a chain reaction. The chain propagation process is suggested to be a reaction of the intermediate Rh(II)TMP with the solvent and the base to yield Rh(I)TMP and an additional reducing radical, (CH3)(2)CO-. No chain reaction was observed with [KOH] < 0.01 mol L(-1) or when the solvent was MeOH. With pyridine as an axial ligand, reduction occurs on the porphyrin ring to yield Rh(III)TMP.-, which then yields the stable Rh(I)TMP. Further radiolytic reduction of Rh(I)TMP in 2-PrOH takes place at the porphyrin ring to yield the chlorin and more highly reduced products.