Quartz solubility in H2O-CO2 fluids was measured at 800 °C and 10 kbar. Mixed fluids were generated from hydrous oxalic acid, silver oxalate, silver carbonate, and liquid H2O; solubility was determined by weight changes of the quartz crystals. Stringent blank tests and weighing procedures were used to establish the CO2 and H2O contents of experimental fluids. Using experimentally constrained models of H2O activity and mixing of silica monomers and dimers, a logarithmic plot of monomer activity vs. H2O activity yields a linear data array of constant slope n insignificantly different from 4 (n = 4.024, R2 = 0.997), where n is the total monomer hydration number. Moreover, all high-quality quartz solubility data in H2O-CO2 fluids at lower temperature and pressure fall on the same line when SiO2 and H2O activities are calculated with this formulation. The same analysis for the dimer yields slope n of 7 (n = 7.049, R2 = 0.996). Our results show that neutral silica monomers and dimers have fixed stoichiometry of Si(OH)4·2H2O and Si2O(OH)6·4H2O (i.e., two solvating H2O molecules per Si) in H2O-CO2 fluids for a large range of crustal metasomatic processes. This provides a simple basis for modeling H2O-CO2-SiO2 fluids over a range of pressure, temperature, CO2, and SiO2 activity.