The activity coefficients of sodium chloride in the NaCl + NaBF4 + H2O ternary system were experimentally determined at 298.15 K, at ionic strengths of 0.3. 0.5, 1, 2 and 3 mol kg(-1) from emf from the bi-ISE cell without liquid junction: ISE-Na|NaCl(m(A)), NaBF4(m(B))|ISE-C| The activity coefficients were fit to the Harned, Scatchard, Pitzer and Lim-HOLL models and compared to those of similar systems such as NaCl + NaClO4 + H2O and NaCl + NaNO3 + H2O. Other related magnitudes, such as the activity coefficients in trace quantities, osmotic coefficients, activity of water, and change in excess Gibbs energy of mixing, were evaluated and discussed in terms of ion-ion and ion-solvent interactions. (C) 2008 Elsevier B.V. All rights reserved.