Al2O3 is estimated to total 4 to 5 mol% in all mantle compositional models, and is believed to be incorporated into (Mg, Fe)SiO3-perovskite at lower mantle conditions. Using synchrotron X-ray diffraction, we have measured the 300 K equation of state of a perovskite with XAl=Al/(Al+Mg+Si)=0.077, up to 32 GPa. A least squares refinement of two independent data sets yields the following equation of state parameters V0=163.52(3) ¿3, K0=229(4) GPa, and K0=2.5(4). These values allow us to assess the most probable substitution mechanism for aluminum into perovskite at pressure conditions of the shallow lower mantle. Al3+ is likely to be incorporated in place of Si4+ in the octahedral site of perovskite, hence requiring the creation of oxygen vacancies for charge balance. As a consequence, aluminous perovskites may have a high affinity for water. The enhanced compressibility of aluminous perovskite certainly has also strong geophysical and geochemical implications, as it may revive perovskite-rich lower mantle models. ¿ 2001 American Geophysical Union