In order to prepare low resistance Ohmic contacts to p-ZnSe and p-GaN by “deposition and annealing (DA)” technique which has been extensively used for GaAs-based devises, formation of an intermediate semiconductor layer (ISL) with high doping concentrations is required. For p-ZnSe, although the metal/p-ZnSe reaction is mandatory to prepare a heavily doped layer, the reaction induced an increase in the compensation donors in the p-ZnSe epilayer close to the contact interface. This indicated that preparation of low resistance Ohmic contacts by forming a heavily doped ISL between p-ZnSe and metal was extremely difficult by the DA technique. On the other hand, Mg acceptors in p-GaN were thermally stable during annealing at temperatures as high as 800 °C. In addition, annealing in oxygen and nitrogen mixed gas reduced the specific contact resistances (ρ_c) of the contact and resistivities (ρ_s) of the p-GaN epilayer after annealing at temperatures of 500 °C. The reason was believed to be due to formation of ISL with high hole concentrations close to the p-GaN surface, caused by removal of hydrogen atoms which bonded with Mg atoms. This excellent stability allows us to anneal the metals deposited on p-GaN at high temperatures without degrading the epilayer quality, which is one of big advantages for p-GaN to prepare Ohmic contacts by DA technique.