This paper reports on the evolution of the structural and electrical proprieties of Au/Ni contacts to p-type gallium nitride (GaN) upon thermal treatments. Annealing of the metals in an oxidizing atmosphere (N2/O2) led to better electrical characteristics, in terms of specific contact resistance (ρc), with respect to non-reacting ambient (Ar). The evolution of the metal structures and the formation of a NiO layer both at the sample surface and at the interface with p-GaN were monitored by several techniques such as transmission electron microscopy (TEM), x-ray diffraction (XRD), and atomic force microscopy (AFM). Furthermore, a reduction of the Schottky barrier height from 1.07 eV (Ar annealing) to 0.71 eV (N2/O2 annealing) was determined by the temperature dependence of the ρc. Local current maps by conductive AFM demonstrate the role of the interface in the conduction mechanism. These electrical results were …