We have determined the quartz-coesite transition by reversed experiments in a piston-cylinder apparatus in the range 500–1200 °C. The difference between the sample pressure and apparent pressure was calibrated by (1) studying the friction decay in the hysteresis loop defined by the relationship between apparent (“nominal”) pressure and piston position in the compression and decompression cycles, and (2) determining the melting temperature of LiCl by DTA in pressure cells similar to those used in the reversal experiments and comparing the results with those determined in the gas apparatus. The equilibrium transition boundary can be expressed as P (kbar) = 21.945 (±0.1855) + 0.006901 (±0.0003)T (K). It is subparallel to, but ∼1.5 kbar higher than, the transition boundary determined by Bohlen and Boettcher (1982). We have also retrieved the entropy [39.56 ± 0.2 J/(mol·K)] and enthalpy of formation (−907.25 ± 0.007 kj/mol) from elements of coesite at 1 bar, 298 K, from our phase-equilibrium data and selected thermochemical data from the literature. From the characteristics of the hysteresis loop we conclude that the often-used practice of maintaining a constant nominal pressure by repeated pressure adjustment during an experiment leads to variation of pressure on the sample.