A flow-through electrochemical cell was developed for direct pH measurement of supercritical aqueous solutions, based on our previously reported cell [Sue et al., Rev. Sci. Instrum. 72 (12) (2001) 4442]. The cell consisted of two high-temperature and high-pressure flow-through platinum hydrogen electrodes. The distance between tips of the two electrodes was minimized to reduce the streaming potential and to simplify the experimental procedure. Flow characteristics were considered in the design by placing orifices near the center of the cell to reduce concentration fluctuations around the electrodes. With the newly-designed cell, pH measurements were made for HCl + NaCl aqueous solutions at flow rates from 0.5 to 6.0 g/min, temperatures from 19.6 to 392.9degreesC and pressures from 0.1 to 29.8 MPa. Comparison of the experimentally determined and theoretically calculated pH was better than +/-0.02 logarithmic units over the range of conditions studied. (C) 2003 Elsevier B.V. All rights reserved.