Ti/Al (10 nm/100 nm) bilayer contacts on Si-doped GaN were heat treated for 30 s at 600 and 650 degreesC. The microstructure of the contacts was analysed by Auger depth profiling, SEM and EDX analysis and energy filtering TEM using electron spectroscopic imaging (ESI). The experiments show a strong interdiffusion of Al and Ti leading to the formation of an intermetallic compound. A complex, spatially inhomogeneous structure consisting of an Al-rich phase, a Ti-Al intermetallic compound and regions containing Al-oxide is formed, while the GaN is still covered by a thin, homogeneous Ti-rich layer. A thickness of 2.5 mu was obtained for the Ti-rich layer in the contact heat treated at 650 degreesC. The combination of complementary results from Auger depth profiling, SEM analysis with EDX and ESI provides a comprehensive characterisation of the complex contact structure. The results will be discussed in terms of the contributions and specific advantages of the individual techniques.