DIFFUSION in silicates plays a key role in a number of processes in the Earth’s mantle, including viscous flow(1-4), electrical conductance(5-8) and the homogenization of chemical heterogeneities. Although cation diffusion rates have been measured in olivine at high pressures(9,10), no data exist on the chemical transport properties of the silicate phases thought to predominate in the transition zone of the mantle (from 400 to 700 km depth). Here we present measurements of cation diffusion in the alpha-olivine phase and high-pressure beta- and gamma-spinel phases of Mg2SiO4 at pressures up to 14 GPa. We find that diffusion rates in both high-pressure phases are about three orders of magnitude faster than that of olivine. When coupled with convective thinning, these faster diffusion rates suggest that the transition zone is more efficient at mixing chemical heterogeneities than the olivine-dominated upper mantle. Furthermore, we calculate that the minimum size of chemical heterogeneities in the transition zone should be of the order of metres.