The potential difference of the cell containing two ion-selective electrodes (ISE), Na-ISE|NaNO3(m), PEG 4000(w), H2O(1 - w)|NO3-ISE, has been measured at temperatures of (288.15, 298.15, and 308.15) K as a function of the mass fraction w of poly(ethylene glycol) 4000 (PEG 4000) in a mixed solvent. w was varied between 0 and 0.25 in 0.05-unit steps, and the molality of the electrolyte (m) was between (0.004 and 5.800) mol·kg-1, approximately. The values of the standard potential difference, E0, were determined using routine methods of extrapolation, together with extended Debye-Hückel and Pitzer equations. The results obtained produced good internal consistency for all the temperatures studied. Once E0 was determined, the mean ionic activity coefficients for NaNO3 were calculated.