pH of 10(-3) mol kg(-1) H2SO4 aqueous solutions at subcritical and supercritical conditions was measured at temperatures from 375.0 to 400.6degreesC and pressures from 25.8 to 32.5 MPa by a flow-through electrochemical cell and it changed front 3.90 at 375.2degreesC and 32.5 MPa to 6.37 at 399.8degreesC and 28.1 MPa. The molal equilibrium constant for the dissociation reaction of H2SO4 was determined based on the measured pH and the value varied front 1.67 x 10(-7) at 399.8degreesC and 28.1 MPa to 4.53 x 10(-4) at 375.2degreesC and 32.5 MPa. The values were between the literature values determined from UV-Vis spectroscopy and electrical conductivity. An empirical density model for the description of the temperature and pressure dependence of the equilibrium constants was used to correlate the data. (C) 2004 Elsevier B.V. All rights reserved.