The concentrations of the species CO32−(aq), NaCO3−(aq), and HCO3−(aq) and the solubility of quartz in 4.65 m NaHCO3 and 1.6 m Na2CO3 solutions have been determined at temperatures (T) to 600°C and pressures (P) up to 1.53GPa using Raman spectroscopy and a Bassett-type hydrothermal diamond-anvil cell. Most Raman spectra of the 1.6 m Na2CO3 fluid and the spectra of the 4.65 m NaHCO3 solution at T⩾400°C showed evidence of Na+–CO32− contact ion pairs. In the 1.6 m Na2CO3 solution, the molal fractions αHCO3- and αNaCO3- of the species HCO3−(aq) and NaCO3−(aq) increased with temperature. At vapor pressure during first heating, HCO3−(aq) was not detectable at 23°C and NaCO3−(aq) not at 23–200°C, and αHCO3- was 0.106 at 200°C. After equilibration with quartz at high temperature, these fractions were higher and showed little dependence on pressure (e.g., αHCO3- was ∼0.13 at 23°C, ∼0.29 at 200°C, ∼0.39 at 400°C, and ∼0.50 at 600°C, and αNaCO3- ∼0.11 at 23°C, ∼0.23 at 200°C, ∼0.34 at 400°C, and ∼0.41 at 600°C). Consequently, αCO32-, the fraction of CO32−(aq), decreased strongly with temperature to ∼0.09 at 600°C. Carbon dioxide was not detected in preceding tests. In contrast to the Na2CO3 solution, significant CO2 formed in runs with H2O+NaHCO3 via autoprotolysis 2 HCO3−(aq)=CO32−(aq)+CO2(aq)+H2O. In the 4.65 m NaHCO3 solution, the HCO3−(aq) fraction decreased with temperature and increased nonlinearly with pressure along all studied isotherms from 200 to 600°C (e.g. at 600°C from 0.66 at 0.63GPa to 0.75 at 1.1GPa to 0.80 at 1.5GPa), whereas αNaCO3-, αCO32-, and αCO2 showed the opposite behavior. These results indicate that HCO3−(aq) is a significant oxidized carbon species in aqueous fluids in the lower crust and upper mantle. The only detectable Raman band from dissolved silica was from symmetric Si–OH stretching of monomers and consisted of two components assigned to the species SiO(OH)3–(aq) and Si(OH)4(aq). The calibrated normalized integrated intensity of this band at ∼770cm−1 was used to determine the SiO2(aq) molality in the fluid. These data showed that the quartz solubility in 1.6 m Na2CO3 increased with P and T, and was always much higher than the solubility of quartz in water at the same P–T condition. The quartz solubility in 4.65m NaHCO3 increased with temperature. At lower pressures, it was generally higher than in water, which indicates a basic pH. Along the 500 and 600°C isotherms, the quartz solubility decreased with pressure to values below that in water.