Using the liquid-liquid partition method, the formation of Co(II) complexes with 1-alkyl-2-methylimidazoles (where alkyl = C4H9 trough C12H25) at 25 degrees C and at a fixed ionic strength of the aqueous phase (I = 0.5, (HL)NO3, KNO3) were studied. Dichloromethane, trichloromethane, and 2-ethylhexanol were used as diluents. The tetrahedral and octahedral complexes were formed. Stability constants (beta(n)) of the complexes as well partition ratios (P-n) of the extracted species were determined. It was shown that both beta(n) and P-n increased with an increasing 1-alkyl chain length. Tetrahedral complexes are more readily extractable by organic solvent. Their P-n values are the highest.