Green defect emission is commonly observed in ZnO nanostructures. It is frequently attributed to oxygen vacancies and used to evaluate performance and study physical mechanisms in a variety of applications, such as gas sensing and photocatalysis. However, competing hypotheses have been proposed to explain green emission, which raises questions about the role of oxygen vacancies in sensing and photocatalytic processes. The major problem in correct experimental identification of defects in ZnO is the abundance of defects present, while theoretically there are problems with accurate calculation of a defect energy level in the gap. Thus, here we adopted a different approach and studied experimentally and theoretically the interaction of ZnO with different chemical substances (hydrogen and a silane-based molecule). Based on theoretical predictions and experimental results, we can conclude that green emission can likely be assigned to defect complexes, which may contain zinc vacancies. (C) 2013 Elsevier B.V. All rights reserved.