A quasichemical method that combines ab initio treatment of explicit solvent with dielectric continuum models has been used to study the origin of a strong effect of methanol on the extent of iron(III) [tetrakis(pentafluorophenyl)]porphyrin chloride dissociation in acetonitrile-methanol solutions. It is shown that the dissociation is energetically more favorable in methanol than in acetonitrile primarily because of the strong specific interactions between the chloride anion and the solvent methanol molecules in its first solvation shell. These interactions are weaker in acetonitrile. The final estimate for the difference in the dissociation free energies in methanol and acetonitrile is -23 kJ/mol, in a good agreement with the experimental value of -21 kJ/mol. Energy decomposition analysis of chloride-solvent interactions suggests that stronger chloride-methanol binding is a result of the contribution of charge delocalization effects to the chloride-methanol interactions.