The adsorption of H-2 and H2O on differently pretreated beta-Ga2O3 samples, exhibiting variable surface chemistry and surface defect concentration, was studied by temperature-programmed reduction. oxidation, and desorption; Fourier transform infrared spectroscopy; and electric impedance measurements. Adsorption of H-2 at 300-473 K resulted in the formation of surface -OH groups with no formation of oxygen defects. Between 473 and 550 K, formation of Ga-H species was observed. At above 550 K, oxygen vacancy/defect formation was observed, resulting in enhanced formation of Ga-H species, accompanied by the formation of more strongly bound Ga-H species. H2O Was observed to quench oxygen vacancies formed by reductive pretreatment and to exhibit a strong hydrolytic affinity to H adsorbed on Ga sites. The effect of water was seen even under strongly reducing conditions (773 K: 1 bar H-2) with only traces of water present. (c) 2008 Elsevier Inc. All rights reserved.