The thermodynamics and kinetics of NO transfer from S-nitrosotriphenylmethanethiol (Ph3CSNO) to a series of alpha,beta,gamma,delta-tetraphenylporphinatocobalt(II) derivatives [T(G)PPCoII], generating the nitrosyl cobalt atom center adducts [T(G)(PPCoNO)-N-II], in benzonitrile were investigated using titration calorimetry and stopped-flow UV-vis spectrophotometry, respectively. The estimation of the energy change for each elementary step in the possible NO transfer pathways suggests that the most likely route is a concerted process of the homolytic S-NO bond dissociation and the formation of the Co-NO bond. The kinetic investigation on the NO transfer shows that the second-order rate constants at room temperature cover the range from 0.76 x 10(4) to 4.58 x 10(4) M-1 s(-1), and the reaction rate was mainly governed by activation enthalpy. Hammett-type linear free-energy analysis indicates that the NO moiety in Ph3CSNO is a Lewis acid and the T(G)PPCoII is a Lewis base; the main driving force for the NO transfer is electrostatic charge attraction rather than the spin-spin coupling interaction. The effective charge distribution on the cobalt atom in the cobalt porphyrin at the various stages, the reactant [T(G)PPCoII], the transition-state, and the product [T(G)(PPCoNO)-N-II], was estimated to show that the cobalt atom carries relative effective positive charges of 2.000 in the reactant [T(G)PPCoII], 2.350 in the transition state, and 2.503 in the product [T(G)(PPCoNO)-N-II], which indicates that the concerted NO transfer from Ph3CSNO to T(G)PPCoII with the release of the Ph3CS center dot radical was actually performed by the initial negative charge (-0.350) transfer from T(G)PPCoII to Ph3CSNO to form the transition state and was followed by homolytic S-NO bond dissociation of Ph3CSNO with a further negative charge (-0.153) transfer from T(G)PPCoII to the NO group to form the final product T(G)(PPCoNO)-N-II. It is evident that these important thermodynamic and kinetic results would be helpful in understanding the nature of the interaction between RSNO and metal porphyrins in both chemical and biochemical systems.