The first two series of Co-NO bond dissociation enthalpies in benzonitrile solution were determined for 12 cobalt(II) nitrosyl porphyrins and for 12 cobalt(Ill) nitrosyl porphyrins by titration calorimetry with suitable thermodynamic cycles. The results display that the energy Scales of the heterolytic Co-III-NO bond dissociation, the homolytic Coli-NO bond dissociation, and the homolytic Coll-NO bond dissociation are 14.7-23.2, 15.1-17.5, and 20.8-24.6 kcal/mol in benzonitrile solution, respectively, which not only indicates that the thermodynamic stability of cobalt(II) nitrosyl porphyrins is larger than that of the corresponding cobalt(III) nitrosyl porphyrins for homolysis in benzonitrile solution but also suggests that both cobalt(III) nitrosyl porphyrins and cobalt(II) nitrosyl porphyrins are excellent NO donors, and in addition, cobalt(III) nitrosyl porphyrins are also excellent NO+ contributors. Hammett-type linear free energy analyses suggest that the nitrosyl group carries negative charges of 0.49 +/- 0.06 and 0.27 +/- 0.04 in T(G)(PPCoNO)-N-II and in T(G)(PPCoNO)-N-III, respectively, which indicates that nitric oxide is an electron-withdrawing group both in T(G)(PPCoNO)-N-II and in T(G)(PPCoNO)-N-III, behaving in a manner similar to Lewis acids rather than to Lewis bases. The energetic and structural information disclosed in the present work is believed to furnish hints to the understanding of cobalt nitrosyl porphyrins' biological functions in vivo.