The properties of standard Ni-Au contacts to p-type GaN were intensively investigated using various microscopic techniques: scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The results of these investigations allowed the basic structural properties to be established after the typical annealing procedures. The distribution of chemical species in the contact structure was investigated using in-depth X-ray photoelectron spectroscopy (XPS). It has been observed that during annealing in an ambient atmosphere containing oxygen and nitrogen, two types of contact structure may develop, depending on the morphology of the (0 0 0 1) GaN surface: (i) relatively low resistivity contact, obtained on a "finger-like" surface and (ii) high resistivity contacts on the GaN surface covered with hillocks containing dislocations. These studies were used to formulate a model, which explains the basic mechanism of contact formation. It is also shown that the non-uniformity in the Au layer could be the principal source of high resistivity of this type of contact. (C) 2010 Elsevier Ltd. All rights reserved.