The activities of water in the KI + KNO3 + H2O System were determined with an electronic hydrometer. The experimental study was carried out at 298.15 K and constant ionic strengths of 2.0, 2.5, and 3.0 mol.kg(-1). Different ionic strength fractions of KNO3, y(B) = M-B/(M-A + M-B), were studied for each ionic strength. The experimental values were fit to three thermodynamic models (Pitzer, Scatchard, Lim); each of these models required only two fitted parameters in order to obtain deviations within experimental uncertainty. In addition, mean ionic activity coefficients were determined for KI and KNO3 as well as the Gibbs excess energy of mixing.