Laser photolysis of nitritomanganese(III) tetraphenylporphyrin, (MnTPP)-T-III(ONO) (1), in degassed toluene gives manganese(II) tetraphenylporphyrin, (MnTPP)-T-III (2), plus NO2. The quantum yield, Phi, for the formation of (MnTPP)-T-II is dependent on the excitation wavelength : Phi = 0.045 at 355nm and Phi =0.0064 at 532 nm. Continuous photolysis studies reveal that the quantum yield for the net photodecomposition of 1 is much smaller (10(-4)). Thus, (MnTPP)-T-II produced by laser photolysis of (MnTPP)-T-III(ONO) mostly returns to 1 by recombination with NO2 according to second-order kinetics with a rate constant 2.2 x 10(9) M-1 s(-1). The kinetics for the reactions of (MnTPP)-T-II with O-2, NO, and pyridine were investigated in detail. In aerated toluene, (MnTPP)-T-II reversibly reacts with oxygen to yield the dioxygen adduct, MnTPP(O-2) (3). The rate constants, k(f)(O-2) and k(b)(O-2), for the formation and dissociation of 3 at 300 K were determined to be 1.93 x 10(7) M-1 s(-1) and 9.0 x 10(4) s(-1), respectively, and the equilibrium constant, k(f)(O-2)/k(b)(O-2), is therefore 2.1 x 10(2) M-1. (MnTPP)-T-II reacts with pyridine to give (MnTPP)-T-II(Py) with a rate constant of 9.5 x 10(8) M-1 s(-1). (MnTPP)-T-II reacts with NO to yield nitrosylmanganese porphyrin, (MnTPP)-T-II(NO), with the rate constant 5.3 x 10(8) M-1 s(-1). The association reactions of (MnTPP)-T-II with NO, pyridine, O-2, and NO2 are discussed in comparison with those of other metalloporphyrins.