The influence of compositional buoyancy on slab descent through the transition zone is studied in a two-dimensional, semidynamical model of plate subduction. The viscosity is temperature- and depth-dependent. Plate motion is imposed by a time-dependent velocity boundary condition including retrograde trench migration. High rates of trench rollback favor stagnation of the slab in the transition zone, and low values favor deep penetration. Buoyancy forces arise from thermal expansion, from phase boundary deflection at 660 km depth, and from density differences between normal mantle and both basaltic and harzburgitic material in the slab. An anomalous density deficit makes basaltic rock buoyant between 680 km and 900 km. In each case, results with and without chemical buoyancy are compared. Its relative importance depends on the plate age. For a 100 m.y. old plate the compositional effect is found to be negligible. For younger plates it does not lead to qualitatively new modes of slab deformation but causes the slab to stagnate near 660 km already at lower trench velocity. However, penetration into the lower mantle is not inhibited even for a 25 m.y. old plate and for very strong chemical buoyancy when the trench is nearly stationary. A section of 24 km thick crust, intended to simulate an oceanic plateau, is found to pass into the deep mantle along with a penetrating slab. Slab break off occurs only for 36 km thick plateau crust. Although the influence of compositional buoyancy is not negligible, it does not seem to be the main controlling factor for the fate of slabs.¿ 1997 American Geophysical Union