Thermal and photochemical reactions of nitroaquacobalt(III) tetraphenylporphyrin, (NO2)(H2O)(CoTPP)-T-III, have been investigated in toluene solutions containing triphenylphosphine, Pphi(3). It is found that Pphi(3) thermally abstracts an oxygen atom from the NO2 moiety of (NO2)(H2O)(CoTPP)-T-III with a rate constant 0.52 M-1 s(-1), resulting in the formation of nitrosylcobalt porphyrin, (NO)CoTPP. The 355-nm laser photolysis of (NO2)(H2O)(CoTPP)-T-III at low concentrations Of Pphi(3) (<1.0 x 10(-4) M) gives (CoTPP)-T-II and NO2 as intermediates. The recombination reaction of (CoTPP)-T-II and NO2 initially forms the coordinately unsaturated nitritocobalt(III) tetraphenylporphyrin, (ON-O)(CoTPP)-T-III, which reacts with Pphi(3) to yield nitro(triphenylphosphine)cobalt(III) tetraphenylporphyrin, (NO2)(Pphi(3))(CoTPP)-T-III. Subsequently, the substitution reaction of the axial Pphi(3) with H2O leads to the regeneration of (NO2)(H2O)(CoTPP)-T-III. From the kinetic studies, the substitution reaction is concluded to occur via a coordinately unsaturated nitrocobalt(III) porphyrin, (NO2)(CoTPP)-T-III. At higher concentrations of Pphi(3) (>4 x 10(-3) M), (NO2)(H2O)(CoTPP)-T-III reacts with Pphi(3) to form (NO2)(Pphi(3))(CoTPP)-T-III: the equilibrium constant is obtained as K = 4.3. The X-ray structure analysis of (NO2)(Pphi(3))(CoTPP)-T-III reveals that the P-Co-NO2 bond angle is 175.0(2)degrees and the bond length Co-NO2 is 2.000(7) Angstrom. In toluene solutions of (NO2)(H2O)(CoTPP)-T-III containing Pphi(3) (>4 x 10(-3) M), the major light-absorbing species is (NO2)(Pphi(3))(CoTPP)-T-III, which yields (NO)CoTPP by continuous photolysis. The laser photolysis of (NO2)(Pphi(3))(CoTPP)-T-III gives (CoTPP)-T-II, NO2, and Pphi(3) as initial products. The NO2 molecule is suggested to be reduced by Pphi(3) to yield NO, and the reaction between NO and (CoTPP)-T-II gives (NO)CoTPP. The quantum yield for the photodecomposition of (NO2)(Pphi(3))(CoTPP)-T-III is determined as 0.56.