The effects of boiling Aqua Regia (AQ), N-2/Cl-2 Plasma followed by AQ and 0, plasma followed by AQ surface treatments prior to Ni/Au (20 nm/20 nm) metallization to p-GaN:Mg (similar to 3 x 10(17) cm(-3)) have been investigated. N-2/Cl-2 plasma was employed in a bid to lower the Ga/N and O/Ga ratios of the GaN surface to improve the contact properties to p-GaN, while O-2 plasma was employed as an alternative to incorporate O into the Ni/Au system. Results show that a low Ga/N ratio does not necessarily correspond to a better contact. The positive effect of O-2 over N-2 anneal is observed only for the AQ-treated sample, although the mechanisms responsible for its positive effect: NiO formation and Ni/Au layer-reversal were observed for all O-2-annealed contacts. We conclude that the effect of O-2 anneal on the Ni/Au contact is dependant on the p-GaN surface prior to metallization. (c) 2006 Elsevier B.V. All rights reserved.