The core-mantle boundary is a fundamental compositional discontinuity in the Earth, where molten iron alloy from the core meets solid silicate minerals from the mantle. Heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle. At the same time, chemical reactions may create a layer of different composition and density than the overlying mantle. Using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell, we investigate how a layer forms at the base of the mantle. Development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core-mantle boundary and on the rate at which the material diffuses into the mantle. If the material is less than 3--6% denser than the overlying mantle, assuming reasonable choices of lower mantle thermodynamic parameters, the reactant material will be swept away by upwelling plumes. More dense material, on the other hand, forms an internally convecting layer that entrains material from above. The varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle. ¿ American Geophysical Union 1993