Activity coefficients of sodium bromide in aqueous mixtures of sodium bromide and Lithium bromide were determined by e.m.f. measurement at T = 298.15 K and at six different ionic strengths from (0.1 to 2.5) mol . kg(-1). The experimental results were fitted using the Scatchard-Rush-Johnson and Pitzer models. The osmotic coefficients, excess Gibbs free energies of mixing, and activity coefficients of lithium bromide in aqueous mixtures were calculated using Pitzer mixing parameters obtained in this work.