The temperature increase across the thermal boundary layer at the base of the Earth's mantle (D″) may be as high as 1000--1300 ¿C, while the excess temperature of mantle plumes, inferred from petrological studies, is ca. 200--300 ¿C. These two estimates are generally considered separately, although this should not be the case if plumes do originate from D″. This paper investigates the evolution of plume excess temperature from the source region in the deep mantle to lithospheric depths. A finite element model in axisymmetric geometry is used to explore the effects of a number of model variables. The results show that if the source region is only a thermal boundary layer, the plume excess temperatures are unrealistically high, since entrainment of surrounding mantle does not cool significantly a large plume head rising through the mantle. I then investigate the role of a chemically denser layer at the base of D″, considering a range of values for its thickness and excess density. The presence of a 30 km thick layer, 5% chemically denser than the surrounding mantle, buffers the excess temperature of plumes to plausible values. The results suggest that chemical stratification across D″ may govern the excess temperature of mantle plumes.¿ 1997 American Geophysical Union