Department of Chemistry, Faculty of Science, Assiut University, Sohag, Egypt Manuscript received 7 April 1989, accepted 7 June 1989 Heterocyclic Schiff base complexes of CoIII are synthesised. The Schiff bases are tridentate and have dinitrogen and oxygen donors. SoIubilities and transfer chemical potentials of perchlorate, thiocyanate and nitrate salt of the (OIII complexes in watermethanol mixtures are reported. Transfer chemical potentials for the CoIII Schiff base cations are calculated Trends with solvent composition suggest that preferential solvation varies from negligible to very strong by methanol depending on the ligand structure and hydrophilic/hydrophobic character of the inorganic ions. Kinetics of reactions between hydroxide ion and the Schiff base complexes of CoIII in aqueous methanol mixtures follow a rate law with Kobs -K1+K2 [OH] or Kobs -K2[OH-] The analyses of the reactivity trends to initial and transition state show that the small increase of the rate constant is ascribed to a greater effect of solvent on the transition state.