Although velocities of subducting slabs should be controlled primarily by their negative thermal buoyancies, their mineralogy can also have significant effects. We explore this by using thermo-kinetic modeling to predict mineralogy and compare the resultant buoyant (driving) force to the opposing viscous drag. Phase transitions of (Mg, Fe)2SiO4 in subducting slabs depend on thermal structure in two ways. First, equilibrium phase boundaries should be deflected, causing local buoyancy anomalies whose sign depends upon that of the Clapeyron slope. As slabs first enter the transition zone, negative anomalies should accelerate them, but positive anomalies that arise when they fully penetrate the transition zone should slow them. Such effects may induce geologically abrupt changes in plate motions. Second, olivine that persists metastably in slabs will form regions of positive buoyancy which should reduce slap velocities. The coldest and fastest slabs should be slowed more greatly, thus narrowing the range of feasible subduction rates. Decreased descent rates, however, allow slabs to warm and metastable wedges to thermally erode. Such negative feedback mechanisms may serve to regulate subduction rates. ¿ 1999 American Geophysical Union