The steady-state dissolution kinetics of quartz in water has been measured at temperatures from 20 to 435 degrees C at pressures of 23 and 33 MPa using a continuous stirred-tank reactor. Dissolution rates were found to increase with increasing temperature from 20 to 374 degrees C at 23 MPa, giving an Arrhenius expression of log k(+) = -1.6478 - 13,573.1/T(K), but then decreased on further increasing the temperature beyond 374 degrees C. At 33 MPa, the rates increased with temperature up to 400 degrees C, giving an Arrhenius expression of log k(+) = -1.325 - 14,124/T(K), but then decreased on further increasing the temperature. At a constant temperature below 350 degrees C, quartz dissolution rates were found to increase with increasing pressure. At around the critical state of water, the dissolution rates fluctuated as a result of sharp changes in the properties of the medium (e.g., density, dielectric constant, ionic product). (C) 2015 Elsevier B.V. All rights reserved.