The microstructural properties and interdiffusion reactions of some common metallization schemes (Au/Ge/Ni, Ti/Pt/Au, WSix, and AuBe) on GaN and In0.5Ga0.5N have been examined using scanning electron microscopy and Auger electron spectroscopy. The objective of this study was to investigate thermally stable and uniform contacts to both materials. The WSix contacts were found to possess excellent thermal stability and retained good structural properties at annealing temperatures as high as 800 degrees C on GaN. These contacts on In0.5Ga0.5N had a minimum specific contact resistivity of 1.48 x 10(-5) Omega cm(2) and an excellent surface morphology following annealing at 700 degrees C. The increase in contact resistance observed at higher temperatures was attributed to intermixing of metal and semiconductor. By contrast the Ti/Pt/Au and Au/Ge/Ni contacts showed lower stabilities, but also showed little Au penetration into the underlying GaN. AuBe contacts had the poorest thermal stability of all four schemes investigated, with substantial reaction with GaN occurring even at 400 degrees C. The WSix contact appears to be an excellent choice for application in high temperature GaN electronics.