Densities and viscosities of water ( 1) + 1-butyl-3-methylimidazolium tetrafluoroborate ( 2) were measured over the entire mole fraction range from ( 303.15 to 353.15) K. From these data, excess molar volumes ( V-E), and viscosity deviations (Delta eta) were calculated. The V-E and Delta eta were fitted to the Redlich-Kister equation using a multiparametric nonlinear regression method. Estimated coefficients of the Redlich-Kister equation and standard deviation calculated from the Redlich-Kister equation to the experimental data are also presented. The results show that the densities and viscosities are dependent strongly on water content. Comparatively, the viscosity deviation Delta eta is more sensitive to temperature than the excess molar volume V-E.