The mean ionic activity coefficients of NaBr were experimentally determined in formamide + water mixtures at 298.15 K from potential difference measurements of the following electrochemical cell containing two ion selective electrodes (ISEs): Na-ISE vertical bar NaBr (m), formamide (w), H(2)O (1-w)vertical bar Br-ISE. The molality (m) varied between 0.02 mol.kg(-1) and almost saturation, whereas the mass fraction of formamide in the mixture (w) was varied between 0 and 1 in 0.1-unit steps. The determination of E(0)* (molal scale) was carried out following a method similar to that proposed by Hitchcock and using the classical extended Debye-Huckel and Scatchard equations. We also use for this purpose a modification of the Pitzer equation proposed by Rard and Archer and the most recent modified three-characteristic-parameter-correlation model. The results obtained produced good internal consistency, within the normal limits of experimental error in these types of measurement. Once E(0)* was determined, the mean ionic activity coefficients of NaBr (gamma), the Gibbs energy of transfer of the NaBr from the water to the formamide + water mixture (Delta G(t)(0)), standard solubility product of NaBr in formamide + water (K(sp)(0)), and NaBr primary hydration number (n(hyd)) were estimated. The results were comparatively analyzed with those of NaF and NaCl previously obtained in similar mixtures.