The thermal behavior of Co(TPP)(L), Co(Tp-CH3PP)(L), and Co(Tp-OCH3PP)(L) was investigated in the solid state, where TPP, Tp-CH3PP, and Tp-OCH3PP represent tetra-phenylporphyrin, tetra(p-methylphenyl)porphyrin, and tetra(p-methoxyphenyl)porphyrin, respectively, and L represents a series of 4-substituted pyridines. Upon heating, these complexes release a pyridine molecule with a structural change from square-pyramidal five-coordination to square-planar four-coordination. The thermal stability (peak temperature of the DSC curves taken as the reaction temperature) was found to be high when the basicity of the bound pyridine is high or the substituent on the pyridine is large in size. The thermal stability is also correlated to the enthalpy change for this reaction. These results were explained by a difference in the Co-pyridine bond strength and by the packing forces for the pyridine in the crystal lattice. Among the three types of porphyrins, the substituent of porphyrin did not affect the thermal stability, probably because of the greater differences in lattice energy.