Electrical conductivities, kappa, of sodium tetrafluoroborate solutions in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) ionic liquid were measured in the wide temperature range of (238.15 to 458.15) K. Additionally, 0 their densities, rho, viscosities, eta, and molar conductivities, A, are reported for the temperature range (278.15 to 358.15) K, supplemented by dielectric data at 298.15 K. The values for and kappa are well-described by the Vogel-Fulcher-Tammann equatior. Walden plots, log(A) vs log(eta(-1)), for the NaBF4 solution; coincide with the straight line found for neat [bmim][BF4], indicating that the solute has only limited impact on the structure of the ionic liquid. This is corroborated by the similarity of the standard molar volume of NaBF4 and its intrinsic volume, which suggests that solute-induced electrostriction is weak. Within the experimental uncertainty, the dielectric properties of the most concentrated NaBF4 solution (0.1739 mol.kg(-1)) were found to be identical with those of pure [bmim][BF4].