Conductivities 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 Y(NO3)(3) + H2O, Ce(NO3)(3) + H2O, and Nd(NO3)(3) + H2O at (293.15, 298.15, and 308.15) K The measured conductivities were used to test the generalized Young's rule and the semi-ideal solution theory. The comparison results show that the generalized Young's rule and the semi-ideal solution theory can yield good predictions for the conductivities of the ternary electrolyte solutions, implying that the conductivities of aqueous solutions of (1:3 + 1:3) electrolyte mixtures can be well predicted from those of their constituent binary solutions by the simple equations.