The [Co(acacen)(PMe2Ph)]ClO4 (.) H2O (acacen = bis(acetylacetone)ethylene diimine) was synthesized and characterized. The equilibrium constants and the thermodynamic parameters were measured spectrophotometrically for 1:1 adduct formation of [Co(acacen)L]ClO4 (.) H2O, where (L = PBu3, PMe2Ph) as acceptors with H2NR (R = n-propyl, n-butyl, sec-butyl) as donors, in ethanol and butanol solvents in constant ionic strength (I = 0.01 M NaClO4) and at various temperatures T = (288 to 308) K. The trend of the reactivity of five-coordinate cobalt(III) Schiff base complexes toward a given amine donor according to the axial ligand is as follow: PMe2Ph > PBu3. Also the trend of the reactivity of the donors (amines) toward a given cobalt(III) Schiff base complex is as follow: n-propylamine > n-butylamine > sec-butylamine. The trend of the reactivity of a given cobalt(III) Schiff base complexes toward a given amine donor according to the solvent is as follow: butanol > ethanol. (C) 2004 Elsevier Ltd. All rights reserved.