In situ observations of the ilmenite-perovskite transition in MgSiO3 were carried out in a multianvil high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300--1600 K was determined to be P (GPa)=28.4(¿0.4)-0.0029(¿0.0020)T (K) based on Jamieson's equation of state of gold [Jamieson et al., 1982> and P (GPa)=27.3(¿0.4)-0.0035(¿0.0024)T (K) based on Anderson's equation of state of gold [Anderson et al., 1989>. The consistency of our results, using Jamieson's equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Anderson's equation of state implies that the depth of the 660-km seismic discontinuity does not match the pressure of this transition. ¿ 2001 American Geophysical Union