UV-vis spectroscopy and conductivity measurement techniques were used to study the physicochemical and structural properties of the binary or ternary mixtures of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) + organic solvent and 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim] [BF4]) + organic solvent systems. The solvents involved were acetonitrile, water, ethanol, ethyl acetate, and tetrahydrofuran. It was indicated that the micropolarity and the aggregation behavior of the mixtures depend strongly on the dielectric constants of the solvents and the composition of the mixtures. (c) 2006 Elsevier B.V. All rights reserved.