The electromotive force E, of the cell containing the ion-selective electrodes Na-ISE \ NaF(m), formamide (Y), H2O(100 - Y) \ F-ISE has been measured at 298.15 K as a percentage of the weight of the amide in the mixed solvent. The percentage (Y) was varied from 0 to 90% in 10-unit steps, and the molality of the electrolyte (m) from approximately 0.005 to saturation. The corresponding standard electromotive forces, E (molal scale). were determined using traditional methods of extrapolation (Debye-Huckel and Pitzer equations). The results obtained by application of both equations show experimental errors of the same sort as those for this type of measurements. Once the E-0 was obtained, we determined the mean ionic activity coefficients for NaF, the transfer free energy from water to the water-formamide mixture, and the NaF primary hydration number. The variation of these parameters, with the composition of this epsilon-increasing solvent, is discussed in comparison with that previously obtained for the epsilon-decreasing methanol-water and ethanol-water systems, in terms of both ion-solvent and ion-ion interactions and of the effect of the medium dielectric constant change. (C) 2004 Elsevier B.V. All rights reserved.