In this paper, the formation of ohmic contacts to n-GaN using Al, Ti/Al and Ti/Au has been investigated. n-GaN Ti/Al contacts showed the lowest specific contact resistance after annealing at 900 degrees C for 30 s. For p-GaN, Ni/Au contacts were studied. A lowering of sheet resistance upon annealing in the case of n-GaN and an increase for p-GaN might be attributed to formation of N-vacancy. No native oxide was detected on GaN surface by XPS.