A nonisothermal, solid state electrochemical sensor for monitoring hydrogen or oxygen dissolved in high temperature (to about 500 K) water is described. Nafion(R) in the form of tube was used as the electrolyte. The reference electrode at ambient temperature and pressure was a saturated aqueous solution of Fe2+ and Fe3+ sulphates. Sensor voltage measurements with dissolved hydrogen showed good reproducibility and long term stability. A regression equation for the hydrogen concentration related to cell voltage and water temperature was derived. Sensor voltage measurements with dissolved oxygen showed relatively poor reproducibility and long term stability. This is attributed to the greater complexity of electrode reactions involving oxygen and water.