Seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested. These studies, employing waves reflected and converted at the slab/mantle interface, suggest that a velocity contrast of 5–10% occurs over a distance of 10–20 km to depths of at least 350 km. We have investigated whether such sharp contrasts can result from thermal structure, compositional differences, phase changes, or other effects such as preferred mineral orientation. Using a mantle and slab temperature model, a petrologic model of mantle and slab mineralogy, and a data base of elastic properties of mantle and slab phases, theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints. At 9.6 GPa and 1000°C (conditions modeled at the slab/mantle interface at ∼280-km depth), cooler temperatures in the slab interior produce, at constant composition, only approximately a 1.75% Vp increase into the slab in any of the peridotitic and eclogitic mineralogies investigated. Variation in Vp among the different bulk compositions is only approximately 0.5%. These contrasts are substantially lower than those obtained from the seismological studies and indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts. We propose that phase transformations in mantle and slab mineralogies, notably elevation of the olivine-spinel phase change normally occurring at 400-km depth, are important factors in these sharp contrasts. Other contributing effects may include preferential mineral orientation near the slab surface and, at shallower depths, the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface.