Formation of Cu(II) complexes of 1-alkyl-2-ethylimidazoles (where alkyl = propyl, butyl, pentyl, hexyl, and octyl) has been studied by using the liquid-liquid partition method, at 25 degrees C and a fixed ionic strength of the aqueous phase (I = 0.5; (HL)NO3, KNO3). The complexes were extracted with 2-pentanone, 2-butanol, isoamyl alcohol, 2-ethyl-1-hexanol, dichloromethane, trichloromethane, and toluene. The length of the 1-alkyl group and the nature of solvent have been shown to influence the extraction process. Extraction curves (log D-M vs. pH) are displaced towards lower pH's with increasing chain length of the 1-alkyl substituent and donor number of the solvents. Stability constants of the complexes in aqueous solution were determined as well as their partition ratios between the aqueous and organic phase. The stability of the Cu(II) complexes increased with increasing 1-alkyl chain length. The stability constants are comparable with 6, ones for the Cu(H) complexes of 1-alkyl-2-methylimidazoles, but smaller than those of the Cu(II)-1-alkylimidazole counterparts. The P-c-partition ratios of the 1-alkyl-2-ethylimidazole complexes with Cu(II) are high, and increased with increasing I-alkyl chain length and the donor number of the solvents. Both the strong steric effect of the ethyl substituent at position 2 and the bulkiness of the 1-alkyl-2-ethylimidazole molecules as well as the strong electron-donating proper-ties of the solvent molecules have an effect on the change of the coordination number of Cu(II) from 6 to 4. The 4-coordinate Cu(II) complexes (distorted tetrahedron) are more readily extractable by organic solvent than are the 6-coordinate ones and for this reason their partition constants, P-c, are high. This finding offers the possibility of extraction of the Cu(II) ions from a mixture cations.