Densities were measured for the ternary systems Y(NO3)(3) + Ce(NO3)(3) + H2O, Y(NO3)(3) + Nd(NO3)(3) H2O, and Ce(NO3)(3) + Nd(NO3)(3) + H2O and their binary subsystems at (293.15, 298.15, and 308.15) K. The results were used to test the applicability of simple equations for the density of the mixed solutions. The predictions are in good agreement with measured values, implying that the densities of the examined electrolyte solutions can be well predicted from those of their constituent binary solutions by the simple equations.