A significant chemical stability has allowed us to perform photoelectrochemical measurements on the semiconducting state of the GdMg hydride system. With electrical impedance spectroscopy we confirm that GdMg hydride in the transparent state is an n-type semiconductor. The donor density was estimated to be 5 3 1021 cm23. Impedance measurements give information about the potential distribution at the semiconductor/solution interface. Photocurrent was observed in the transparent state. It is proposed that photoanodic etching of the GdMg hydride at high light intensities occurs along grain boundaries, leading to inhomogeneous destruction of the film.