Isopiestic osmotic coefficients were determined for the unsaturated and NaNO3-saturated H2O + NaNO3 + Y(NO3)(3) + La(NO3)(3) system up to the maximum ionic strength I = 22 mol.kg(-1) at 298.15 K. The experimental results were represented by a modified Pitzer model (J. Phys. Chem. B 2007, 111, 3704). The isopiestic data were found to closely fit a modified Zdanovskii-Stokes-Robinson model for the unsaturated system and Wang's ideal-like solution model (Ben Bunsen-Ges. Phys. Chem. 1998, 102, 1045) for the NaNO3-saturated quaternary system, respectively, which indicate that there is nonzero interchange energy between NaNO3 and each rare earth nitrate and a zero interchange energy between the unlike rare earth nitrates. The isopiestic osmotic coefficient measurements and the Pitzer model representations for the NaNO3-saturated quaternary systems are in good agreement with the ideal-like solution model.