Abstract The spatial distribution of hydrogen in Zircaloy-2, which was oxidized in H 2O steam at 723 K and quenched down to room temperature, was examined by a technique of tritium autoradiography. Tritium was introduced into the specimen through the oxide layer by the cathodic charging method using tritium water at room temperature. The radiograph of the cross-section was observed by a scanning electron microscope. Hydrogen was densely distributed on the grain boundary and in the grain of the inner base alloy, but it was scarcely distributed in the oxide layer. Hydrogen was locally distributed, however, in the region corresponding to the intermetallic precipitates embedded in the oxide layer. The mechanism of penetration of tritium through the oxide layer during the cathodic charging was discussed.