Abstract The viscosity measurements for some symmetrical tetraalkyl ammonium bromide (R4NBr, R = CH3 to C5H11) KCl, and KBr solutions in acetonitrile (ACN) + water mixtures containing 0, 10, 30, 50, 70, 90, and 100 mass% of ACN at 303.15 K have been performed. The Jones–Dole equation was used for the analysis of viscosity data of these salts. The viscosity B-coefficients in the entire range of ACN+water mixtures were obtained from the computerized least-squares method. The experimental criterion B(K+) = B(Cl−) = (1/2)B(KCl) was applied in order to obtain the ionic B-coefficients. The B± coefficients are interpreted in terms of solvent structure and ion-solvent interactions in the solvents.