The [Co(Chel)(PBu 3 )]ClO 4 .H 2 O(Chel = dsp, mdsp and ndsp, where dsp = N,N'-disalicylidene-1,2-phenylene diamine, mdsp = 4-methyl-N,N'-disalicylidene-1,2-phenylene diamine and ndsp = 4-nitro-N,N'-disalicylidene-1,2-phenylene diamine), were synthesised and characterised. The equilibrium constants and the thermodynamic parameters of these complexes as acceptors with PBu 3 , were measured spectrophotometrically for 1 : 1 adduct formation in various mixed EtOH/H 2 O and constant ionic strength (I = 0.1M NaClO 4 ) and at various temperatures T = 288-308 K. The trend of the reactivity of five coordinate Co(III) Schiff base complexes toward PBu 3 donor according to the Schiff base ligand is mdsp > dsp > ndsp. The trend of the reactivity of a given Co(III) Schiff base complexes towards PBu 3 donor according to the water/ethanol mole fractions is 0.12 > 0.36 > 0.52 > 0.64.