The densities of tetraalkylammonium bromide, R4NBr (R = CH3 through n-C5H11), solutions in (0, 10, 30, 50, 70, 90, and 100) mass % acetonitrile + water have been measured over the whole composition range at 298.15 K. From these densities, apparent and limiting partial molar volumes of the electrolytes and ions in these mixtures have been evaluated.