In this paper, we demonstrate that it is possible to know the refractive index, n(D), of every given mixture of 1-alkyl-3methyl imidazolium tetrafluoroborate with water and ethanol just from the knowledge of the refractive index and density of pure components. To do that, we measured nD for seven different mixtures in all range of existing concentrations and, independently, we deduce nD theoretically. Both sets of values differ less than a 0.2% on average. The theoretical deduction takes into account that these mixtures are quasi-ideal from the molar volume point of view, as recently published, and so density for any composition of the mixture can be obtained with a precision better than 0.5% from the pure compounds value. Now we simply apply Newton or Gladstone-Dale models, which relate the refractive index of a binary mixture with its density from the value of both pure components, without any fitting parameter. Both models are very similar in form and in the values they deduce ( less than a 0.2% of difference), but while that of Newton performs slightly better for ethanol mixtures, the model of Gladstone-Dale gives some better results for aqueous mixtures. We think that these results can be extended to the majority of ionic liquid plus solvent systems. (C) 2011 Elsevier Ltd. All rights reserved.