We have developed a flow-through electrochemical cell (FTEC) and demonstrated that it operates successfully in both subcritical and supercritical aqueous environments at temperatures up to 400 degrees C. The FTEC consists of a previously developed flow-through external (Ag\AgCl) pressure-balanced reference electrode (FTEPBRE) (Lvov et al., J. Electroanal. Chem. 444 (1998) 186) and a modified flow-through platinum hydrogen electrode (FTPHE). The experimental measurements have been carried out for two solutions: (1) 10(-3) mol kg(-1) HCl + 10(-1) mol kg(-1) NaCl and (2) 10(-2) mol kg(-1) HCl + 10(-1) mol kg(-1) NaCl at flow rates between 0.1 and 1.4 cm(3) min(-1) and at temperatures of 25 degrees C (18.7 MPa), 200 degrees C (23.8 MPa), 350 degrees C (24.2 MPa), and 400 degrees C (25.3 MPa). We have also carried out a detailed irreversible thermodynamic analysis of the FTEC and estimated different significant contributions to the measured potentials. Comparison of experimentally observed and theoretically calculated pH values shows an ability to use the FTEC for high precision potentiometric and pH measurements. We conclude that high temperature pH measurements with accuracy better than +/-0.03 logarithmic units can now be accomplished. Also, we assume that valuable streaming potential data can be obtained using the FTEC for estimating the surface charge (zeta potential) on the oxide material capillary tube in hydrothermal solutions at temperatures up to 400 degrees C.