The X-ray powder diffraction technique has been used to determine the average ionic radius of Cen+ present in the Nd2-xCexCuO4 electron superconductor system. The effect of the ionic size of the dopant M on the c-lattice dimension of a lanthanide-doped Nd2-xMxCuO4 system has been determined from a similar Nd2-xThxCuO4 electron superconductor system in which the ionic radius of the Th4+ ions is known. Values of the c-lattice dimension obtained from the experimental data of Nd2-xCexCuO4 (x=0.0-0.20) samples are within the upper and lower bounds of the c-lattice dimensional calculated assuming pure Ce3+ (r=1.143 A ̊) and pure Ce4+ (0.97 Å) ions, indicating an intermediate ionic radius and hence an effective valence between +3 and +4. Values of the average ionic radius and the effective valence obtained from our Nd2-xCexCuO4 samples are 0.998 Å and +3.84, respectively. © 1989.