Ce- and Th-doped Nd2CuO4+y superconducting and related compounds were systematically synthesized and characterized. An X-ray diffraction analysis of the variation of lattice dimensions of the Nd2-xMxCuO4+y compounds revealed that Ce had an average ionic radius of 0.998 Å and an effective valence of +3.84, significantly different from those of the formal Ce3+ and Ce4+ ions. An iodometric titration analysis detected excess oxygen above the nominal value of 4 in both the Ce- and Th-doped materials. The analysis also showed that the average [Cu-O]p charge reversed from positive to negative as x increased, and reached a negative maximum x=0.15 where superconductivity appeared. Using effective valences of +3.84 for Ce and +4 for Th ions, the values of p were found to be -0.067 and -0.048 for the 21 K Nd1.85Ce0.15CuO4.030 and the 15 K Nd1.85Th0.15CuO4.048 electron superconductors, respectively. © 1989 IOP Publishing Ltd.