Solvent-corrected reduced isotropic spectra of carbonate and bicarbonate in light and heavy water have been measured from 150 to 325 degrees C at 21 MPa using a confocal Raman microscope and a custom-built titanium flow cell with sapphire windows. The positions of the symmetric vibrational modes of CO32- and HCO3-/DCO3- were compared to density functional theory (DFT) calculations with a polarizable continuum model in light and heavy water. The experimental Raman peak positions shifted linearly toward lower wavenumbers with increasing temperatures. Raman scattering coefficients, measured relative to a perchlorate internal standard, were used to determine equilibrium molalities of the carbonate and bicarbonate species. These yielded quantitative thermodynamic equilibrium quotients for the reaction CO32- + H2O (sic) HCO3- + OH- and its deuterium counterpart. Ionization constants for HCO3- and DCO3-, K-2a,K-H,K-m and K-2a,K-D,K-m, calculated in their standard states using the Meissner-Tester activity coefficient model, were combined with critically evaluated literature data to derive expressions for their dependence on temperature and pressure, expressed as solvent molar volume, over the range 25 to 325 degrees C from p(sat) to 21 MPa. These are the first experimental values to be reported for this reaction in light water above 250 degrees C and in heavy water above 25 degrees C. The value of the deuterium isotope effect on the chemical equilibrium constant, Delta pK(2a,m) = pK(2a,D,m) - pK(2a,H,m), decreased from Delta pK(2a,m) = 0.67 +/- 0.07 at 25 degrees C to Delta pK(2a,m) = 0.17 +/- 0.13 at 325 degrees C and p(sat).