An extensive X-ray study of CeO2-Nd2O3 solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO1.5 were measured using several techniques. Solid solutions containing 0-80 mol% NdO1.5 were synthesized by coprecipitation from Ce(NO3)(3) and Nd(NO3)(3) aqueous solutions, and the coprecipitated samples were sintered at 1400 degrees C. A fluorite structure was observed for CeO2- which changed NdO1.5 solid solutions with 0-40 mol% NdO1.5 to a rare earth C-type structure at 45-75 mol% NdO1.5. The change in the lattice parameters of CeO2-NdO1.5 solid solutions, when plotted with respect to the NdO1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C-type regions. The maximum solubility limit for NdO1.5 in CeO2 solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0-70 mol% NdO1.5) of solid solution.