Subducted oceanic lithosphere consists of a sandwich of component layers (MORB underlain by depleted parent mantle). These layers give rise to intrinsic density contrasts within the slab which have been hypothesized to cause component separation after subduction. We explore this process through numerical models of simple layered slabs and through convection models which incorporate large, local chemical density contrasts. We show that the components of positive and negative buoyancy (density contrast x component volume) must be almost equal in order to cause significant component separation or to selectively block deep penetration of buoyant slab components in the transition zone. Published petrological constraints on component densities and relative volumes indicate that such buoyancy neutralization is not achieved in subducted slabs. Therefore, component separation should not occur on the timescale associated with the sinking of slabs through the upper mantle and transition zone. ¿ American Geophysical Union 1989