CaSiO3 perovskite is the third most abundant mineral in the Earth's lower mantle; it also occurs as a major phase at lower pressures and temperatures in subduction zones, where, for example, it may form ~25 wt% of subducted mid-ocean ridge basalt. Its crystal structure, ferroelastic phase transitions, elastic properties and equation of state are, therefore, of great significance for the interpretation of seismological data. The phase diagram of CaSiO3 perovskite is currently in a state of utter confusion, with computer simulations and experimental studies disagreeing widely as to the structures of CaSiO3 and the P/T conditions under which they occur. Recent work at the ESRF has shown that the introduction of 0.5–1.0 wt% of H2O stabilises a non-cubic phase to lower P & T. We propose to use PEARL to make an accurate structure determination H2O-doped CaSiO3 perovskite at 1250 K and 10 GPa.