We investigated the structural behavior of the Ni/Au contact on GaN(000l) during annealing in N2, using in-situ x-ray diffraction, anomalous x-ray scattering, and high resolution electron microscopy. Thermally activated atomic mobility caused the two metal atoms, Au and Ni, to interdiffuse during annealing and form solid solutions. At temperature higher than 500°C, GaN decomposition and reactions occurred mostly along GaN dislocations. By decomposed nitrogen reacted with Ni, interestingly, epitaxial Ni4N phase was formed. The epitaxial relationship of the Ni4N, Au, and Ni was identified as M(111)//GaN(0002) and M[0 1 1]//GaN (M= Ni4N, Au, and Ni).