We investigated the reaction chemistry of metal contacts to GaN during annealing using X-ray photoelectron spectroscopy (XPS). GaN decomposition was estimated, using XPS, to occur in N-2 annealed Ni-alloy contacts at 550 degreesC. The reaction was greatly accelerated by the catalytic effect of Au and Pt. The decomposition was correlated with the rapid degradation of electrical properties during annealing. The results suggest that high-temperature applications may be critically limited by the degradation of metal contacts especially due to activated Ni reactivity in Ni-alloyed contacts. Meanwhile, the thermal stability of Ni/Au contact greatly improves by suppressing the activated Ni reactivity, which is able to be obtained by forming preferential Ni-O bonding through annealing in air.