In the present work the impedance spectroscopy and secondary ion mass spectrometry (SIMS) were used to study the oxygen/beta -Ga2O3 interaction between 578 and 835 degreesC. The overall resistance of the beta -Ga2O3 is composed of the bulk resistance and the grain boundary resistance. If oxygen is present, dominantly the characteristics of the space charge region - formed at the grain boundaries - change. The mechanism of the bulk conduction is unchanged between 578 and 835 degreesC, however, the processes determining the grain boundary resistance are different above and below similar to 700 degreesC. At low temperatures the formation and adsorption of O-2(-) ions is probable, while at higher temperatures the presence of O- ions is dominant on the grain boundaries.