The stability of the electrical and structural properties of GaAs directly deposited on Si by metalorganic chemical vapor deposition is examined. Extended annealing at 900 °C leads to substantial diffusion of Si across the heterointerface while under the same conditions there is no significant motion of Si incorporated as a dopant into the GaAs surface region. The degree of enhancement of Si diffusion ranges from a factor of ∼250 for 0.5-μm-thick GaAs films to ∼5 for 4-μm-thick films. The annealing time and GaAs layer thickness dependence of Si diffusivity near the interface is consistent with a defect-modulated mechanism. A large fraction of this mobile Si is electrically inactive.