The activity coefficients of NaBF4 in water were experimentally determined at 288.15, 298.15, and 308.15 K from emf measurements of the following electrochemical cell without transference containing two ion selective electrodes (ISE): Na-ISE vertical bar NaBF4(m)vertical bar BF4-ISE The molality m varied between ca. 0. 1 and 9 mol . kg(-1). The Hitchcock method of extrapolation and the models of Pitzer and Scatchard were used to obtain the mean ionic activity coefficients from measurements of electromotive force (emf). The electrode system employed demonstrated good Nerstian behaviour. The activity and osmotic coefficients were calculated using the Pitzer parameters obtained in this study, and were contrasted with those from the literature which had been determined only at 298.15 K. Finally, these activity coefficients are discussed together with other sodium salts. (C) 2007 Elsevier Ltd. All rights reserved.