The polycrystalline structure of the superconducting Nd2-xThxCuO4 system is studied using the x-ray diffraction technique. High quality x-ray powder diffraction data are obtained, and analysis shows that the Nd2-xThxCuO4 compounds synthesized by solid state reaction are well-crystallized and have a body-centered tetragonal T′-structure. A minor amount of Cu2O is detected suggesting a possible Cu deficiency in the x≥0.10 compounds. A significant amount (in the 10% range) of unreacted ThO2 is also found in the x=0.25 sample which confirms the Th solubility limit in Nd2-xThxCuO4 is 0.20. New x-ray powder diffraction data with excellent figures of merit of F30>210 and M20>260 of the superconductor Nd1.85Th0.15CuO4 and its parent compound Nd2CuO4 are reported. Values of the lattice dimensions with an estimated precision of 2.5×10-5 by least-squares refinements from 32 to 33 reflections in the front reflection region are obtained. Decreases in the c lattice dimension and the c/a ratio with increasing x are caused by the substitution of smaller Th4+ ions for larger Nd3+ ions. © 1989.