A novel solid state electrochemical sensor having a heterojunction structure consisting of a solid proton conductor and a solid oxygen-ion conductor has been developed to measure hydrogen dissolved in aluminum melt. The sensor measures hydrogen concentration with reference to an oxygen at a reference compartment. To fix the oxygen partial pressure, two different methods were used. Either a mixture of CuO and Cu2O or else oxygen gas in air were employed to maintain a constant oxygen partial pressure at the reference electrode. The sensing mechanism of the heterojunction sensor is discussed and the characteristics of the sensor are reported in a hydrogen gas atmosphere at temperature ranging from 650 to 800 degrees C. The heterojunction hydrogen sensor was packaged and dipped into aluminum melt to measure hydrogen content during gassing and degassing processes. (C) 2016 Elsevier B.V. All rights reserved.