Phase transformations in diopside have been studied at pressures up to 25 GPa and at temperatures of 1100 and 1500 ¿C by a multiple anvil apparatus. The following phase assemblages are found to be stable at given pressures; diopside (15 and 18 GPa), Mg2SiO4 spinel+stishovite+CaSiO3 perovskite (19 and 20 GPa), MgSiO3 ilmenite+CaSiO3 perovskite (22 GPa), and MgSiO3 perovskite+CaSiO3 perovskite (24.5 GPa). Electron microprobe analysis reveals that the solubility of the MgSiO3 molecule in CaSiO3 cubic perovskite is limited to between 2--5%. The solubility of the CaSiO3 molecule in MgSiO3 orthorhombic perovskite is more restricted, being of the order of 2% or less. These results are contradictory to earlier work in which diopside is reported to transform into a cubic perovskite structure of the same composition above 22--23 GPa. ¿ American Geophysical Union 1989