The thermal decompositon (PT) curve of dolomite has been extended in piston-cylinder apparatus from a point at 9 kbar and 1130¿C to 1450¿C. The piston-cylinder runs, in which unsealed Pt sample capsules were embedded in glass pressure media, were compatible with earlier gas system determinations. At approximately 11 kbar and 1175¿C the curve delineating the onset of decomposition of CaMg(CO3)2 deviates strongly from any extrapolated extension of the PT curve up to 10 kbar and 1150¿C, and the colomite composition is stabilized to significantly higher temperatures than those predicted from low-pressure data. At 1450¿C, PCO2 is less than 14 kbar. The striking increase in thermal stability at pressures above 10 kbar is the result of rapidly shifting phase boundaries in TX sections at temperatures above the calcite-dolomite solvus, and probably of an entropy increase in CaMg(CO3)2, due to a variety of disordering mechanisms.