Partial molar volumes at infinite dilution, V-2(infinity), are reported for tetra-n-alkylammonium bromides, R(4)NBr (R = Me to Pen) in x dimethyl sulfoxide + (1-x) water mixtures, up to x = 0.1587, at 298.15 K. A linear dependence between V-2(infinity) of the electrolyte and the molecular weight of the tetraalkylammonium cation was found.