Activity coefficients of sodium bromide in aqueous mixtures of (sodium bromide and tetramethylammonium bromide) were determined by e.m.f, measurements at T = 298.15 K and at eight different ionic strengths from (0.1 to 2.5) mol . kg(-1). The experimental data were fitted using the Scatchard, Fitter, and Lim equations, respectively. The osmotic coefficients, excess Gibbs free energies of mixing, and activity coefficients of tetramethylammonium bromide were calculated using Fitter parameters obtained in this work.