The electromotive force of the cell containing two ion-selective electrodes (ISEs), Na-ISE|NaBF 4 (m), PEG4000 (Y), H 2O (100 - Y)|BF 4-ISE, has been measured at temperatures of (288.15, 298.15, and 308.15) K as a function of the weight percentage Y of PEG4000 (poly(ethylene glycol) 4000) in a mixed solvent. Y was varied between (0 and 25) % in five-unit steps, and the molality of the electrolyte (m) was between ca. (0.04 and 7) mol·kg -1. The values of standard electromotive force were determined using routine methods of extrapolation, together with Debye-Hückel and Pitzer equations. The results obtained produced good internal consistency for all of the temperatures studied. Once the standard electromotive force was determined, calculations were made of the mean ionic activity coefficients for NaBF 4, the Gibbs energy of transfer from the water to the PEG4000-water mixture, and the primary NaBF 4 hydration number.