The development of reliable Ohmic and Schottky contacts on GaN will require an understanding of the thermal stability and metallurgy of metal-GaN contact structures. We investigated the behavior of Pt, Pd, and Ni on GaN as a function of annealing temperature. Rutherford backscattering spectrometry, x-ray diffraction, and scanning electron microscopy were used to characterize the interface and surface behavior. 800Å thin metal films were deposited by sputtering on 2μm GaN films grown by metal organic chemical vapor deposition on sapphire substrates. No reactions occur in the Pt, Pd, and Ni systems with annealing up to 800°C. The Pt film begins to form submicron spheres and islands after annealing above 600°C. The Pd and Ni films begin to island with annealing above 700°C. Below these temperatures no structural changes were observed.