Abstract The coesite—stishovite transition was determined over the temperature range 500–1100° C by means of in-situ X-ray measurements with NaCl as an internal pressure standard. A cubic-anvil type of high-pressure apparatus and a high-power X-ray diffraction system were used for this study. Based on Decker's new pressure scale (1971), the coesite—stishovite transformation curve was represented by the linear equation P(kbar) = (80 ± 2) + (0.011 ± 0.003)(° C). Both the transition pressure at 1000° C and the slope dP/ dT obtained in the present investigation are a little smaller than the previous determinations by the quenching method. Comparison with the thermodynamic data suggests that some uncertainty is still involved in the determination of the entropy of stishovite.