High-temperature and high-pressure UV-vis spectroscopy was applied to evaluate the proton concentration in supercritical water (SCW)-CO2 (3 and 7 mol%) ir ixture at 380 degrees C and a water density of 400kg m(-3) using 4-nitrophenol as a probe molecule. From the absorption spectra measured at various NaOH concentrations, the first dissociation constant of aqueous carbon dioxide (CO2,aq + H2O = H+ + HCO3-) was determined to be 3.3 x 10(-13). The determined dissociation constant was one order of magnitude smaller than that estimated from the revised HKF equation of state (2.5 x 10(-12)). Using the determined dissociation constant, proton concentration in SCW-CO2 mixture was estimated at various CO2 concentrations (1-7 mol%) and was shown to be 6-19 times higher than that in pure SCW. The compound, 4-nitrophenol, was found to be a reliable probe for estimating proton concentrations in supercritical aqueous solutions. The estimated proton concentration in SCW-CO2 mixture can be used for detailed analysis of SCW reactions, especially for acid-catalyzed ones. (c) 2007 Elsevier B.V. All rights reserved.