A double crossing of the perovskite-postperovskite phase transition in (Mg,Fe)SiO3 has been proposed to explain seismic reflections from structures near the base of the mantle. Estimates of temperature inferred from the phase transitions can be extrapolated to the core-mantle boundary (CMB) using a simple model for the thermal boundary layer. However, a complication arises when flow is driven by the (negative) buoyancy of the postperovskite phase. Latent heat release and advective transport increase the temperature gradient and heat flow below the region of postperovskite. A minimum bound on the temperature gradient at the CMB is obtained by noting that the temperature gradient at the base of the postperovskite region must be at least as steep as the transition temperature with depth. Both the temperature and heat flow at the CMB depend on the vertical velocity at the phase transition. A representative velocity of 1 mm/yr gives a local heat flux of 160 m W m−2. Increasing the velocity by a factor of two yields a substantially higher heat flow and an unreasonably hot temperature at the top of the core.