A solid-state hydrogen sensor was fabricated with a 9% yttria-stabilized zirconia (YSZ) disk sandwiched between two platinum films; one of which was completely covered by a catalyst with a composition of 7CuO 10ZnO 3Al(2)O(3). Its hydrogen sensing characteristics in a mixture of nitrogen, oxygen, and hydrogen were examined under limiting current conditions with respect to oxygen. The sensor response was adequately correlated against the hydrogen concentration and the operating temperature according to a proposed theoretical model, which relates the response to the rate processes occurring in the catalyst, the solid-state electrolyte, and the electrodes. The response equation given by the model consists of a first-order and a half-order hydrogen concentration terms. The model shows that the first-order concentration effect would prevail at high hydrogen concentrations using a sensor consisting of a thick compact layer of highly active catalyst and a thick solid-state electrolyte. On the other hand, the present experimental sensor with a thin porous layer of moderately active catalyst and a thin YSZ disk electrode showed, as described by the model, a response of half-order hydrogen concentration dependence in a gas mixture containing 0.018 to 0.18 mole percent of hydrogen.