Scanning photoelectron microscopy (SPEM) and X-ray absorption spectroscopy have been used to study interface reactions between Si and GaN when GaN is deposited on nitridated silicon substrates at high temperatures. The results show that the interface reactions between silicon and GaN are strongly dependent on the reaction temperature with the more severe reactions occurring at higher temperatures. In addition to the observation of the direct reaction of nitrogen with the Si to form silicon nitride at the interface, the chemical maps obtained with SPEM indicate that Si migrates to the top surface of the GaN layers. The migration of Si to the surface results in formation of SiNx on the top surface and in severe roughening of the GaN layers deposited at 950 and 1010 degrees C, X-ray absorption data indicate that the hexagonal structure forms at higher temperature while the cubic form of GaN is the dominant structure at low temperature. The three-dimensional growth of the hexagonal structure appears to be responsible for enhancement of the Si migration.