Using acoustic measurement interfaced with a large volume multi-anvil apparatus in conjunction with in situ X-radiation techniques, we are able to measure the density and elastic wave velocities (V P and V S ) for both ortho- and high-pressure clino-MgSiO 3 polymorphs in the same experimental run. The elastic bulk and shear moduli of the unquenchable high-pressure clinoenstatite phase were measured within its stability field for the first time. The measured density contrast associated with the phase transition OEN -> HP-CEN is 2.6-2.9% in the pressure of 7-9GPa, and the corresponding velocity jumps are 3-4% for P waves and 5-6% for S waves. The elastic moduli of the HP-CEN phase are K S =156.7(8)GPa, G = 98.5(4)GPa and their pressure derivatives are K S '=5.5(3) and G' = 1.5(1) at a pressure of 6.5GPa, room temperature. In addition, we observed anomalous elastic behavior in orthoenstatite at pressure above 9GPa at room temperature. Both elastic wave velocities exhibited softening between 9 and 13-14GPa, which we suggest is associated with a transition to a metastable phase intermediate between OEN and HP-CEN.