Chlorocobalt(III) and iodorhodium(III) tetraphenylporphyrins, (CI)(CoTPP)-T-III and (I)(RhTPP)-T-III, react with 2,6-dimethylphenyl isocyanide (R-NC) in toluene solutions to give the isocyanide complexes, (Cl)(R-NC)(CoTPP)-T-III and (I)(R-NC)(RhTPP)-T-III, respectively. The 355-nm laser photolysis studies have shown that these complexes photodissociate the axial ligand, R-NC. The quantum yield phi, for the photodissociation of R-NC from (CI)(R-NC)(CoTPP)-T-III is determined as 0.091 in both degassed and oxygen-saturated toluene. On the other hand, (I)(R-NC)(RhTPP)-T-III gives phi = 0.19 in degassed toluene and phi = 0.07 in oxygen-saturated toluene. The dissociation of R-NC from (I)(R-NC)(RhTPP)-T-III was confirmed to occur from both the excited singlet and tripler states by transient spectroscopic measurements. The small quantum yield obtained with the oxygen-saturated solution is explained by the quenching of the triplet state, (3)(pi,pi*) of (I)(R-NC)(RhTPP)-T-III. The triplet state decays with a rate constant 9.5 x 10(4) s(-1) in degassed toluene and is effectively quenched by oxygen with a bimolecular rate constant 6.7 x 10(8) M-1 s(-1). In comparison with (I)(R-NC)(RhTPP)-T-III, the lifetime of the (3)(pi,pi*) State of (CI)(R-NC)(CoTPP)-T-III is too short to be detected by the present laser photolysis system, probably because of the very fast radiationless process, (3)(pi,pi*) --> (3)(pi,d(z)(2)) --> (3)(d pi,d(z)(2)). The dissociation of R-NC from (Cl)(R-NC)(CoTPP)-T-III is assumed to occur from the (3)(d pi,d(z)(2)) state. The long lifetime of the (3)(pi,pi*) State of (I)(R-NC)(RhTPP)-T-III is interpreted on the assumption that the (3)(d pi,d(z)(2)) is located higher in energy than the (3)(pi,pi*) state. The dissociation of R-NC from (I)(R-NC)(RhTPP)-T-III at the tripler slate is suggested to occur from the thermally populated (3)(d pi,d(z)(2)) state. The laser photolysis studies of carbonyl-rhodium(III) tetraphenylporphyrin are also described to elucidate the ligand dissociation mechanism of rhodium(III) porphyrins.