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A new water-soluble Co(II) Schiff-base complex, sodium[{N,N 0 -bis(5-sulfosalicylidene)-1,8-diamino-3,6-dioxaoctan}cobalt] dihydrate, abbreviated as Na 2 [Co(II)L], was synthesized and characterized. The formation constants and thermodynamic parameters for the interaction of this complex with imidazole (Im) and 1-methylimidazole (MeIm) were determined spectroscopically in aqueous solution, ethanol/water (10/90), and methanol/water (10/90) under physiological conditions (pH ¼ 7), constant ionic strength (I ¼ 0.1 mol dm À3 KNO 3 ), and various temperatures ranging from 294 to 310 K. Our spectroscopic and thermodynamic results show that this adduct formation is endothermic and the positive values of DS f make DG f negative. The trend in variation of DH f and DS f for Im is in the order water4methanol4ethanol, but for MeIm it is in the opposite order which is related to the hydrogen bonding between solvents and these donors. Formation constants between MeIm and Na 2 [Co(II)L] in these three solvents are larger than for Im which depends on the electron donation of methyl on MeIm.