Densities and viscosities for the binary mixtures of H(2)O (1) + 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][CF(3)SO(3)]) (2) were measured over the entire mole fraction range from (303.15 to 343.15) K at atmospheric pressure. Excess molar volumes and viscosity deviations as a function of mole fraction average were derived, and the results were fitted to the Redlich-Kister equation using a multiparametric nonlinear regression method. Estimated coefficients of the Redlich-Kister equation and standard error calculated from the Redlich-Kister equation to the experimental data are also presented. The results show that the densities and viscosities are dependent on water content and temperature.