The mean ionic activity coefficients of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid in water at a low concentration range were determined using potentiometric measurements at temperatures ranging from 298.15 to 328.15 K. The obtained experimental data were correlated by the Pitzer and nonelectrolyte Wilson nonrandom factor (N-Wilson-NRF) models. The resulted Pitzer parameters were used to estimate the osmotic coefficients and excess Gibbs free energies of the system being investigated. The results indicate that both the models could be applied to predict the activity coefficients of the aqueous ionic liquid solutions, but the Pitzer model gives much lower standard deviations than the N-Wilson-NRF model. Moreover, the obtained activity and osmotic coefficients show that their values decrease with a rise in the ionic liquid concentration and increase with the elevation of temperature.