An advanced palladium/platinum resistance sensor has been developed to measure, in situ, dissolved hydrogen in high temperature aqueous systems. The measurement of hydrogen is based on the well-known fact that the resistance of palladium changes with the absorption of hydrogen into the lattice. Comparative electrical resistance measurements are made between a pair of palladium and platinum probes wound on an oxidized zirconium metal mandrel. Laboratory tests demonstrate that the sensor displays good sensitivity and responsiveness to changes of dissolved hydrogen concentration in an aqueous system at temperatures up to 410 degrees C and at pressures to 4000 psi (276 bar). The sensor is designed for use in water-cooled thermal power plant heat transport circuits and in supercritical-water reactor media for the reductive destruction of toxic waste, where continuous hydrogen monitoring may be necessary.