Chloroaquachromium(III) tetraphenylporphyrin, Cl(H2O)(CrTPP)-T-III, reversibly reacts with isocyanide, CN-R, to form Cl(CN-R)(CrTPP)-T-III in toluene solutions containing 4.4 x 10(-3) M H2O and CN-R. The equilibrium constant for the formation of CI(CN-R)(CrTPP)-T-III obtained by spectroscopic measurements ranges from 10 to 10(2) depending on the nature of the isocyanide used. The laser-photolysis studies of the toluene solution demonstrate that CN-R is photodissociated from Cl(CN-R)(CrPP)-P-III to produce (ClCrTPP)-T-III with a quantum yield ca. 0.1. The five-coordinate (ClCrTPP)-T-III reacts with H2O and CN-R to give Cl(H2O)(CrTPP)-T-III and Cl(CN-R)(CrTPP)-T-III, respectively. The transient product Cl(H2O)(CrTPP)-T-III further reacts with CN-R to regenerate Cl(CN-R)(CrTPP)-T-III. The rates for the decay of Cl(H2O)(CrTPP)-T-III are measured as a function of the concentration of CN-R in order to determine the rate constants of the forward and backward reactions for the formation of Cl(CN-R)(CrTPP)-T-III. The equilibrium constants obtained from the kinetic analysis for the decay of Cl(H2O)(CrTPP)-T-III are in good agreement with those determined by spectroscopic measurements. The mechanism for the equilibrium reaction is discussed in detail on the basis of the rate constants for the formation of Cl(H2O)(CrTPP)-T-III and Cl(CN-R)(CrTPP)-T-III from (ClCrTPP)-T-III, measured in the temperature range 200-300 K. The quantum yield for the photodissociation of CN-R is not affected by the presence of oxygen. Since the lowest excited state T-6(1) Of Cl(CN-R)(CrTPP)-T-III observed by laser photolysis is effectively quenched by oxygen, this state is not responsible for the ligand ejection from CI(CN-R)(CrTPP)-T-III. The S-4(1) State is assumed to be the reactive excited state of Cl(CN-R)(CrTPP)-T-III.