A hydrogen sensor has been developed for in situ measurements of the concentration of hydrogen in aqueous solutions at elevated temperatures. The sensor was based on an electrochemical cell employing an yttria-stabilized zirconia (9% Y2O3) solid electrolyte, a Ag-O-2 (in air) reference electrode, and a noble metal working electrode (Pt or Pd) covered with a polytetrafluorene-ethyl membrane. The response of the sensor to hydrogen has been examined in gas mixtures and aqueous solutions at temperatures as high as 300 degrees C, and for hydrogen concentrations ranging from 0.001 to 100% or 7.61 x 10(-6) to 7.61 x 10(-4) mol/kg, respectively. The sensor displayed rapid responses to changes in concentration of hydrogen in both gas mixtures and aqueous solutions. Linear relationships with Nernstian slopes (2.303RT/2F) were observed between the measured potential of the sensor and the logarithm of the hydrogen concentration. Oxygen in solution had little effect on the potential of the sensor when the concentration of oxygen was lower than that of hydrogen, while there was a significant change in the potential with oxygen concentration in gas mixtures. The potential of the sensor was not affected by the pH of the solution.