Reduction of carbon dioxide emissions and humanity's reliance on fossil fuels by generation of renewable energy and fuel is a potentially effective solution. In this context, the role of oxygen defects and acidity of transition metal oxides during steam reforming of ethanol and hydrodeoxygenation of carboxylic acids are discussed. It is found that steam reforming of ethanol requires more oxygen defects with high mobility but less acidity while biofuel production by hydrodeoxygenation requires relatively strong acidity and/or moderate metal-oxygen bond strength in the transition metal oxides. The finding provides new opportunities for revealing the relationship of structures and catalytic performance, and insights in the rational design of transition metal oxides as catalysts or supports for renewable energy and fuel production. (C) 2014 Elsevier B.V. All rights reserved.