The oxidation of polycrystalline GaN powder and GaN epi layers in dry air has been investigated. Bulk theta-2 theta X-ray diffraction (XRD) revealed no evidence of oxide formation on the powder specimen exposed to temperatures of to 750 degrees C for 25 h. However, when oxidized at temperatures of 900 degrees C or greater for 1 h or longer, an oxide was observed to form and was identified as the monoclinic beta-Ga2O3, the same oxide observed previously by the present investigators to form on epitaxial GaN films. Information regarding the oxidation kinetics was obtained in the present study by measuring the intensity of the beta-Ga2O3, ((2) over bar 1 7) XRD peak as a function of oxidation time at various temperatures, and the initial stage of the oxidation was found to be limited by the rate of an interfacial reaction with an activation energy of similar to 3 x 10(5) J/mol. An interfacial reaction-controlled mechanism was also observed to limit the rate of oxidation of GaN epi layers at 900 degrees C.