The equations of state of end-member MgSiO3 low-clinoenstatite and orthoenstatite have been measured by single-crystal X ray diffraction to pressures in excess of 8.5 GPa. The volume variation with pressure of low-clinoenstatite (space group P21/c) is described by a third-order Birch-Murnaghan equation of state with the parameter V0=31.292(8) cm3 mol-1, KTO=111.1(3.3) GPa, and K'TO=6.6(1.1). At pressures in excess of 7 GPa the low-clinoenstatite inverts to a high-clinoenstatite phase (space group C2/c), with a volume reduction of 0.798(14) cm3 mol-1. This transformation is reversible but with significant hysteresis at room temperature; the low-clinoenstatite is recovered on decompression to below 5.3 GPa. The volume variation of orthoenstatite (space group Pbca) displays a discontinuity at ~4 GPa which corresponds to a change in the internal mechanism of compression. The equations of state fitted to data below and above 4 GPa have the parameters: P4 GPa, V0=31.23(10) cm3 mol-1, KTO=122.8(16.5) GPa, and K'TO=5.6(2.9). From these data, and the slopes of the equilibrium phase boundaries, estimates of ΔS, ΔH, and ΔV of the transformations between the three enstatite polymorphs are calculated. ¿ American Geophysical Union 1994