The activity coefficients of sodium fluoride in the NaF + NaClO4 + H2O ternary system were experimentally determined at 298.15 K, at ionic strengths of 0.1-0.9, in 0.1 mol kg(-1) steps from emf from the bi-ISE cell without liquid junction: ISE-Na/NaF(m(A)), NaClO4(m(B))/ISE-F The activity coefficients were compared with those of the NaCl + NaClO4 + H2O and NaBr + NaClO4 + H2O systems by using the Harried, Scatchard, Pitzer, and Lim-HOLL models. 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. Finally, the solubility of the NaF in the mixtures with NaClO4 were predicted and compared with values collected from the literature, with good agreement obtained. (c) 2005 Elsevier B.V. All rights reserved.