The MgSiO3 orthoenstatite-clinoenstatite (OEn-CEn) phase boundary has been reversed between pressures of 70 and 110 kbar and temperatures of 900 ¿C and 1700 ¿C with a split-sphere anvil apparatus (USSA-2000). Starting materials contained PbO-PbF2 (1:1) flux to promote equilibration of the charges and eliminate a potential effect of deviatoric stresses on the phase boundary. The phase boundary separating orthoenstatite from clinoenstatite at high pressures and temperatures can be described by the equation P (kbar)=0.031 T (¿C)+50. Our results are consistent with previous unreversed determinations of the boundary at these high-pressure, high-temperature conditions. However, the dP/dt slope determined in the present study is much smaller than that implied by earlier experimental studies of the orthoenstatite/low clinoenstatite boundary at low pressures and temperatures. We propose that the clinoenstatite observed at high pressures and temperatures is a new high-pressure clinoenstatite phase which is possibly an analogue of MgGeO3 clinopyroxene. ¿ American Geophysical Union 1990