The chemical thermodynamic concepts used in the calculation of solubility data of silica (quartz) are presented taking into account the PVT characteristics of water. The temperature-dependcnce trends between the thermodynamically calculated and the experimental quartz solubility data are very similar, but the values are widely different at high temperatures. The experimental solubility, especially along the saturation curve at high temperature and thermodynamic data for silica need to be reevaluated in order to use silica chemistry to understand geological processes. There could exist a wide range of values for silica solubility at a specified temperature, depending upon the amount of water in the reaction vessel. Thus the silica contents in geothermal fluid, in general, cannot be used as a geothermometer to estimate the reservoir temperature. The derivation of a silica geothermometer needs an extra assumption about the total amount of water in the system. The solubility data for the two extreme cases, i.e. when the vessel (bomb) is completely filled with water and when there is just enough water to make the total specific volume equal to the critical volume of water at room temperature (25 degrees C), are considered here, These lie on the two respective straight lines of log (SiO2(ppm)) against temperature (K). The equations for the two straight lines are log (SiO2 (ppm)) = 0.0179 T (K)-4.3214 and log SiO2 (ppm) = 0.0088 T (K)-1.6513, respectively. In the case of the well M-19A at Cerro Prieto, the silica concentration in the reservoir liquid is higher than the experimental solubility, but is lower than the calculated solubility value. (C) 2000 CNR.