The impact of phase equilibria and equation of state measurements on defining the buoyancy associated with the 660 km discontinuity is examined. Aluminum has a striking effect on the sequence of phase transitions expected at 660 km. With a pyrolite chemistry the sign of the phase transition contribution to buoyancy depends on temperature, with convection-assisting forces being victorious in hotter regions. Lower Al and/or temperature result in the perovskite-forming transformation resisting convection but with a much reduced magnitude compared with a pure olivine mantle. Density considerations suggest that there is no positive evidence for chemical-induced buoyancy across the 660 km discontinuity. The surface processing which forms a basaltic crust will cause a density contrast that assists convection in the transition zone but resists convection through the expanded garnet stability field of mid-ocean ridge basalt. The detailed seismic character of the 660 km depth region and its lateral variations hold the key to defining the location of the mantle state relative to the parameter space defined by Al and temperature in this study. ¿ 1998 American Geophysical Union