Glancing angle X-ray diffraction and Auger electron spectroscopy have been used to investigate interfacial reactions between the Ni(20 nm)/Pt(30 nm)/Au(80 nm) contacts and p-GaN (p = 9.4 x 10(16) cm(-3)). The metallization schemes were annealed at temperatures ranging from 500 to 700 degrees C for 30 s in a flowing Ar atmosphere. The anneal of the sample at 500 degrees C resulted in ohmic behavior with a specific contact resistance of 2.1 x 10(-2) Omega cm(2). However, the anneal at temperatures greater than or equal to 600 degrees C led to the degradation of the ohmic property. It is shown that Ga-(Pt,Ni) phases such as Ga3Pt5 and Ga4Ni3, and a Pt-Ni solid solution are formed upon annealing at 500 degrees C, while in addition to the Ga4Ni3, new phases of GaAu2 and GaAu are formed upon annealing at temperatures greater than or equal to 600 degrees C. As for the Pr-Ni solid solution, the lattice parameter decreases significantly with increasing annealing temperatures. A qualitative explanation is given to describe why the characteristic of the contacts changed from ohmic to rectifying behavior with increasing temperature.