Numerical simulations of mantle convection are highly sensitive to the dependence of phase relations upon pressure, temperature, and composition. Phase transition in the (relatively silica-poor) olivine components of the mantle tend, with increasing iron content, to inhibit convective mass transfer between the upper and lower mantle. Those occurring in the (relatively silica-rich) non-olivine components fail to inhibit (and indeed promote) such mass transfer. Behavior in the real mantle must entail an interplay between such competing effects, with the finite width of multivariant phase fields acting to mitigiate impediments to flow. Attempts to directly correlate computed flow or temperatures fields with models derived from physical observable should be approched with caution, given the sensitivity of the former to compositional heterogeneity and uncertainties in phase relations. ¿ American Geophysical Union 1995.