An operational electrochemical thermocell is proposed for potentiometric pH measurements in high subcritical and supercritical aqueous solutions. The thermocell consists of the silver-silver chloride flow-through external pressure-balanced reference electrode (FTEPBRE) and the flow-through platinum-hydrogen pH sensor. Using this thermocell, we have measured the potentials of the thermocell for a few HCl(aq) and NaOH(aq) solutions of different concentrations and derived the corresponding pH differences (Delta pH) over a wide range of temperatures from 25 to 400 degrees C at pressures between 27.5 and 33.8 MPa. Comparison of the experimentally derived and theoretically calculated Delta pH values clearly demonstrates an ability of the thermocell to measure pH with accuracy better than +/-0.1 units, and this result in turn demonstrates the viability of the FTEPBRE as a versatile reference electrode for use at high subcritical and supercritical temperatures. (C) 1998 Elsevier Science B.V. All rights reserved.