The lattice parameter changes with respect to temperature (7) have been measured by high temperature X-ray diffraction (HTXRD) technique for ThO2-NdO1.5 solid solutions containing 23.8 and 42.5 mol% NdO1.5 in the temperature range from 298 to 2000 K. The temperature versus lattice parameter data have been made use of in calculating the lattice thermal expansivity. The values of thermal expansion of the solid solutions were found to be increased with increase in neodymium oxide content and temperature. The mean linear thermal expansion coefficients in this temperature range for ThO2-NdO1.5 solid solutions are 12.28 x 10(-6) and 12.90 x 10(-6) K-1, respectively. The binding energies of Th 4f(7/2) and Nd 3d(5/2) energy levels of the solid solutions containing 13.1, 23.8, 31.9, 37.2 and 42.5 mol% NdO1.5 and two-phase mixtures containing 47.6 and 51.8 mol% NdO1.5 were experimentally determined by X-ray photoelectron spectroscopy (XPS). (c) 2006 Elsevier B.V. All rights reserved.