Using first-principles calculations we predict the effects of composition on the perovskite to post-perovskite phase transition. The transition is predicted at 107 GPa for pure MgSiO3. The addition of Al2O3 slightly increases this transition pressure, and the addition of Fe2+ considerably reduces it; the FeSiO3 end-member term is stable in the post-perovskite modification with respect to perovskite at all pressures. We also determine the static equations of state, densities, elasticity and seismic wave velocities. At the transition Vp increases slightly, and Vs increases significantly, consistent with the seismic observations for D″. The addition of both Fe2+ and Al2O3 decrease the seismic wave velocities.