ZnSe has been used as an interlayer between Si substrates and GaAs layers in molecular beam epitaxial growth of GaAs on Si. It is found that thin GaAs layers are much more uniform and have fewer defects when grown on ZnSe interlayers than when they are grown directly on Si. The growth of GaAs on ZnSe is much more difficult than the more usual reverse sequence, and different growth modes for the epitaxy of GaAs on ZnSe are compared. Deposition of GaAs on ZnSe at room temperature followed by solid phase regrowth led to an epitaxial layer plus a polycrystalline layer. A slow ramping of the substrate temperature during the GaAs epitaxial growth was found to give the best crystal quality.