The water-gas shift reaction was studied in either direction on beta-Ga2O3 samples with different surface chemistries: a fully oxidized surface, a hydrogen-pretreated surface with and without oxygen vacancies present, and an oxygen-defective surface without adsorbed hydrogen. The samples were characterized by volumetric adsorption measurements and temperature-programmed desorption and reaction techniques. The reaction in both directions was observed to follow two parallel mechanistic pathways, namely formate- and oxygen vacancy-assisted mechanisms, with relative rate contributions depending on temperature and sample pretreatment. (c) 2008 Elsevier Inc. All rights reserved.