The solubility of natural, near-end-member wollastonite-I (>99.5% CaSiO 3 ) has been determined at temperatures from 400 to 800°C and pressures between 0.8 and 5GPa in piston-cylinder apparatus with the weight-loss method. Chemical analysis of quench products and optical monitoring in a hydrothermal diamond anvil cell demonstrates that no additional phases form during dissolution. Wollastonite-I, therefore, dissolves congruently in the pressure–temperature range investigated. The solubility of CaSiO 3 varies between 0.175 and 13.485wt% and increases systematically with both temperature and pressure up to 3.0GPa. Above 3.0GPa wollastonite-I reacts rapidly to the high-pressure modification wollastonite-II. No obvious trends are evident in the solubility of wollastonite-II, with values between 1.93 and 10.61wt%. The systematics of wollastonite-I solubility can be described well by a composite polynomial expression that leads to isothermal linear correlation with the density of water. The molality of dissolved wollastonite-I in pure water is thenlog(mwoll)=2.2288-3418.23×T-1+671386.84×T-2+logρH2O×(5.4578+2359.11×T-1).By combining the present experimental data with literature data on the solubility of quartz (Manning, C.E., Boettcher, S.L., 1994. Rapid-quench hydrothermal experiments at mantle pressures and temperatures. Am. Mineral. 79, 1153–1158.) and wollastonite-I+quartz (Xie, Z., Walther, J.V., 1993b. Wollastonite+quartz solubility in supercritical NaCl aqueous solutions. Am. J. Sci. 293, 235–255.) in pure water, an analogous expression can be derived for the solubility of wollastonite in quartz-saturated aqueous solution as follows:log(mwoll)=2.5930-3660.98×T-1+671402.32×T-2+logρH2O×(-1.3609+6775.13×T-1).(all temperatures are in°C)This expression adequately describes wollastonite-I solubility between 0.2–3GPa and 300–800°C.