Impedance spectroscopy is used to distinguish the bulk and grain-boundary conduction phenomena in a nonstoichiometric, polycrystalline beta-Ga2O3 thin layer with a grain size of 30 nm, between 567 and 790 degrees C. When oxygen is present, between 567 and 630 degrees C, the width of the depletion layer exceeds the half-diameter of the grains, and the grain is completely depleted. However, as the effective Debye length decreases with increasing temperature, above 630 degrees C, the grain is not completely depleted. The conduction mechanism in the bulk is the same in the whole temperature range studied. The activation energy is determined to be 1.2 eV. However, the grain boundary conduction mechanism changes around 700 degrees C.