This article is a contribution to the modeling of the thermodynamic properties of 15 lanthanide (including lanthanum)-nitrate aqueous binary solutions, from low molalities to saturation (for Ce, Gd, Tb, and Tm) and supersaturation (for La, Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb, and Lu) with respect to the corresponding trinitrate lanthanide solid salts. A critical compilation of the experimental and previous modeling data available in the literature is presented. The modeling approach is based on the standard Pitzer formulation for strong aqueous electrolytes, with two cases considered: three (beta((0)), beta((1)), and C-phi) or five (beta((0))), beta((1)), beta((2)), alpha(2), and C-phi) parameters. The best fits are obtained for the latter case, leading to the representation of unprecedented accuracy for the osmotic and activity coefficients over the whole range of experimental data up to the saturation/supersaturation points. On the basis of the models developed, the thermodynamic solubility product constants and the standard molar Gibbs free energy of formation of precipitating hexahydrate or pentahydrate lanthanide-nitrate solids are calculated.