Abstract: A hydrogen reference electrode is disclosed which is applicable to measuring systems for determining the oxygen content in high temperature water in the range of 400.degree. F. to the critical point of water of approximately 713.degree. F. at which it co-exists in the liquid and vapor states and which water is treated to provide a reducing atmosphere. The reference electrode is formed as a closed end tube of palladium-Silver alloy material pressurized with hydrogen gas and enclosed in a perforated sleeve which allows water to be trapped in the space between the alloy tube and the sleeve where the water is saturated with hydrogen permeating through the wall of the alloy tube.