By means of static ab-initio computations we investigate the influence of Al on the recently discovered perovskite to post-perovskite phase transition in MgSiO3. We examine three substitution mechanisms for Al in the two structures: MgSi → AlAl; SiSiO → AlAl□; and Si → AlH. The substitutions introducing oxygen vacancies (highly unfavorable, energetically) and water (favorable) both lower the 0 Kelvin transition pressure, whereas charge coupled substitution increases it relative to 105 GPa for pure MgSiO3. From the transition pressures for 0, 6.25, and 100 mol% charge coupled Al2O3 incorporation and simple solution theories, we estimate the phase diagram of Al-bearing MgSiO3 at low Al concentrations. Assuming the Clapeyron slope is independent of Al concentration, we find the perovskite-to-post-perovskite transition region to span 127--140 GPa, at 6.25 mol% Al2O3. When the upper pressure limit is bounded by the core-mantle boundary, the phase coexistence region has width 150 km.