Correlations between volumetric properties and refractive index of binary mixtures of room temperature ionic liquids (RTILs) and organic solvents were examined. To this end, the density and refractive index for a set of these systems were measured at atmospheric pressure at 298.15 K throughout the composition range. These data were used to calculate excess volumes and refractive index deviations by using expressions firmly based on the physical significance of each quantity, fact that allowed the expected relations between the two quantities to be confirmed. Based on these results, the molar refraction and free or void volume of the mixtures are calculated with a view to estimating the relative contribution of both quantities to the excess molar volume. Once molar refraction was confirmed to exhibit a near-ideal behaviour in all mixtures, a method for predicting the density and refractive index of RTIL+organic solvent mixtures was developed; the results show that this procedure can be a highly useful alternative to the usually complex experimental methods available for the thermophysical characterization of these systems. (C) 2008 Elsevier Ltd. All rights reserved.