The electromotive force, E, of the cell containing two ion-selective electrodes (ISE), Na-ISE\NaF(m), EC(Y), H2O(100 - Y)\F-ISE has been measured at a temperature of 298.15 K as a function of the weight percentage Y of ethylene carbonate (EC) in a mixed solvent. The Y was varied between 0 and 60 wt.% in 10-unit steps and the molality of the electrolyte (m) was between ca. 0.005 and saturation. The values of the standard electromotive force, E-0 (molal scale), were determined using routine methods of extrapolation (Debye-Huckel and Pitzer equations) and also using a chemical model in order to account for the possible formation of ion pairs (Na+F-)(0). The results obtained produced good internal consistency, within the normal limits of experimental error encountered in these types of measurement. Once Eo was determined, the mean ionic activity coefficients for NaF, the free energy of transfer from the water to the EC-water mixture, and the primary NaF hydration number were calculated. The variation of these magnitudes with the composition of the mixture with epsilon-increasing co-solvent is discussed in comparison with those previously obtained with formamide-water, as well as for those from epsilon-decreasing methanol-water and ethanol-water systems in terms of the ion-solvent and ion-ion interactions and their changes with the properties of the medium. (C) 2004 Elsevier B.V. All rights reserved.