Backscattering spectrometry, x-ray diffractometry, and scanning electron microscopy have been used to study the reaction of a thin Pt film with an epilayer of [110] GaN on [110] sapphire upon annealing at 450, 550, 650, 750, and 800 degrees C for 30 min. A Ga concentration of 2 at. % is detected by MeV He-4(++) backscattering spectrometry in the Pt layer at 550 degrees C. By x-ray diffraction, structural changes are observed already at 450 degrees C. At 650 OC, textured Ga2Pt appears as reaction product. The surface morphology exhibits instabilities by the formation of blisters at 650 degrees C and voids at 800 degrees C.