The viscosities of potassium halides, KCI and KBr, and tetraalkylammonium bromides, R4SBr (R = -CH3 through n-C5H11), in acetonitrile + water mixtures have been measured at 298.15 K and corresponding B-coefficients of the Jones-Dole equation have been calculated. From these B values, the ionic B-coefficients of R4N+ and Br-ions have been determined, assuming that the equality B-K(+) = BCI- holds for the different solvent mixtures.