A flow-through electrochemical cell for subcritical and supercritical water systems was developed based on the cell of Lvov et al. [J. Electroanal. Chem. 463, 146 (1999)]. The cell consisted of two flow-through platinum hydrogen electrodes and was constructed from a high nickel alloy (Hastelloy-B2). The inner surface of electrochemical cell was coated with Al2O3 adhesive and covered with the Al2O3 tube for corrosion resistance. Orifices were placed near the center of the cell to reduce concentration fluctuations around the test electrode. With the cell, a flow apparatus was used to measure the potentials for HCl+NaCl aqueous solutions at supercritical conditions. The cell potentials were measured at temperatures ranging from 23.9 to 400.2 °C and at pressures ranging from 25.0 to 35.1 MPa and the standard deviations of measured potentials were less than 0.4 mV for all tests. Comparison of the experimentally determined and theoretically calculated pH differences are better than ±0.03 logarithmic units over the range of conditions studied.