Isobaric heat capacities of α-quartz, coesite, and stishovite were measured by differential scanning calorimetry at 183--703 K. The heat capacity data of coesite represent a significant revision of the previous data. The heat capacities and entropies were also calculated using Kieffer's model. By high-temperature solution calorimetry, transition enthalpy for the β-quartz-coesite at 979 K was measured to be 1.27¿0.39 kJ/mol. The enthalpies for the α-quartz-coesite and coesite-stishovite transitions at 298 K were measured to be 3.40¿0.56 and 33.62¿1.01 kJ/mol, respectively, by differential drop-solution calorimetry. The enthalpy of the coesite-stishovite transition obtained in this study is about 10--15 kJ/mol smaller than those in the previous studies. Phase relations in SiO2 at high pressures and high temperatures were calculated using these new thermodynamic data. The calculated boundary for the α-quartz-coesite transition is consistent with those determined experimentally. For the coesite-stishovite transition, the calculated boundary having a slope of 2.5¿0.3 MPa/K disagrees with that by Yagi and Akimoto but is generally consistent with that by Zhang et al.'s new in situ X ray diffraction study. ¿ American Geophysical Union 1995