The oxidation-reduction (known as redox) electrode was studied in a pH range corresponding to that found in a CO/sub 2/-silicate buffered system at temperatures ranging from 100 to 250/sup 0/C. Oxidizing conditions were created by injecting oxygen; the greatest reducing conditions, by using hydrogen. The sulfur system (H/sub 2/S, HS/sup -/, S/sup 2/) was not examined at this time, but future work will include this system since it is an important component of the geothermal brine. Both a platinized platinum (Pt-Pt) and a bare platinum (Pt-foil) electrode respond to the redox environment. The response does not appear to indicate thermodynamic equilibrium with the chemical environment; instead a mixed potential response is indicated. The difference in potential between a strong oxidizing environment (0.4-ppM O/sub 2/) and a strong reducing environment (0.5-atm H/sub 2/) is typically > 1 volt at all temperatures. The magnitude of this response makes the platinum electrode a suitable sensor for continuous monitoring of the geothermal environment for oxygen intrusions. The platinized platinum redox electrode is superior to the bare platinum electrode; PNL-3377