Viscosities and refractive indices have been determined for (water + 1-butyl-3-methylimidazolium tetrafluoroborate) and (water + 1-butyl-2,3-dimethylimidazolium tetrafluoroborate) mixtures at 298.15 K, over the whole composition range. The refractive indices were compared with the predictions of the Lorentz-Lorenz, Wiener, and Gladstone-Dale equations. Viscosity deviations (Delta eta) and refractive index deviations (Delta n(D)) have been calculated and fitted to the Redlich-Kister polynomial equations. Delta n(D) are positive whereas Delta eta are negative over the entire mixture composition for the two salts. The influence of the structure of imidazolium cation on the above physicochemical properties was discussed. (C) 2007 Elsevier Ltd. All rights reserved.