The electromotive force E of the cell {NaxHg1-xNaCl(m)AgClAg} has been measured at the temperature 298.15 K as a function of the mole fraction x of Na metal in amalgams and of the molality m of NaCl in (methanol + water) solvent mixtures containing up to 0.8 mass fraction of methanol, with relative permittivities epsilon greater than or equal to 40. The respective standard electromotive forces E(m)(o) have been determined, together with the activity coefficients gamma(+/-) of NaCl as a function of its molality. For interpolation purposes the E(m)(o) values can be expressed as a function of the methanol mass fraction w(M) by the following polynomial, valid for 0 less than or equal to w(M) less than or equal to 0.8 : E(m)(o)/V = 2.18126 - 0.092727 . w(M)(1/2) + 0.103039 . w(M) - 0.24716 . w(M)(3/2). These results for standard e.m.f.s. lead to a primary hydration number n(hydr)approximate to 7 for NaCl which is in good agreement with previous results using different methods.