The stability of GaAs/Si superlattices grown on GaAs substrates using molecular beam epitaxy is described. Typical superlattice structures consisted of ten periods of thin (less than 6.5Å thick) layers of pseudomorphic silicon alternating with thick GaAs layers. We have examined the As2/Ga flux conditions required for the growth of high quality superlattices and have found that the structural perfection is extremely sensitive to the V/III flux ratio. The best superlattices were grown under condition that were just barely enough arsenic to produce a stable (2×4) surface reconstruction in the GaAs layers; increases in the arsenic overpressure resulted in a progressive trend towards 3-D growth of the GaAs on the pseudomorphic Si. In addition, we have examined the stability of GaAs/Si superlattices towards post-growth annealing. Double crystal x-ray diffraction scans showed little change in superlattice structure following rapid thermal anneals at 800°C; at 900°C, however, all but the first order satellite reflections disappeared. We attribute this behavior to the relaxation of pseudomorphic strin and the generation of misfit dislocations at the higher anneal temperature.