There has been an explosion of activity in the field of high-temperature superconductivity since the discovery of superconductivity in Laz_xBaxCuO 4 [1]. Subsequently, superconductivity near 100 K has been found in the layered-perovskite family [1-4]. There are controversies as to whether holes in oxygen play an important role in superconductivity or not. Recently, an n-type superconductor of Ndz_xCexCuO 4 (x = 0.15) showing a critical temperature (To) of 30 K has been reported by Tokura et al. [5]. Superconductivity was also found in fluorine-doped NdzCuO 4 [6]. As well as the doping with fluorine, the introduction of hydrogen into transition-metal oxides has been employed to produce n-type semiconductors [7]. In fact, hydrogendoped YBazCu3O7_~Hy was prepared by the direct reaction of these superconducting oxides with hydrogen gas at a relatively low temperature [8-10]. In this study we attempted to prepare hydrogen-doped NdzCuO4, and the effects of introduced hydrogen on the crystal structure, the formal valence of copper, and superconductivity are examined. Stoichiometric mixtures of Nd203 and CuO were heated to 950 °C in air for 10 h. After the calcination was repeated twice, the mixtures were fired at 1100°C in air for 10h, and slowly cooled in a furnace. The resulting samples were confirmed to be a single phase of NdzCuO 4 by X-ray diffraction (XRD). Hydrogen-doped samples were prepared as follows: tlhe single-phase powders of NdzCuO 4 were heated at 250 °C in 101.3 kPa H 2 for 20 h, and quenched to room temperature. The presence of hydrogen contained in the hydrogen-doped sample was detected by a pulsed hydrogen nuclear magnetic resonance (H-NMR; Bruker PC20C) instrument. In order to obtain the relationship between the hydrogen concentration and the initial intensity of a free induced decay (FID) curve, FID curves were taken for mixtures of ( 1 x) CuO and xCuSO4"5H20, where x ranged from 1 to 0.05. It was confirmed that a linear relationship existed between x and the initial intensity of the FID curves. From this calibration curve, the hydrogen content of the H-doped sample, Nd2CuOxHy, was estimated. The oxygen content of the hydrogen-doped sample was determined by hydrogen reduction using a thermogravimetric analysis system (Shimazu TGA-30); the hydrogen-doped sample was heated in flowing hydrogen (50 cm 3 min -1) to 700 °C at 5 °Cmin -1. The formal oxidation state of copper in the hydrogendoped sample was determined by the iodiometry method. These hydrogen-doped samples were pressed into pellets, and then the superconducting