We present a density-functional theory analysis of stoichiometric and nonstoichiometric crystalline and amorphous Zn-Sn-O systems (c-ZTO, a-ZTO) which connects structural features with electronic properties in order to contribute to the understanding of the recently discovered subgap states in a-ZTO and other amorphous oxide films. In particular we show that defect levels originating from oxygen vacancies are too high in energy to be responsible for levels above the valence band edge. We offer an explanation for the experimentally seen decrease of subgap states with increasing oxygen content. From our analysis of the energetic stability of c- and a-ZTO compounds with different Zn/Sn ratios the decomposition of ZnSnO3 into Zn2SnO4 and SnO2 at sufficiently high temperatures is conceivable. Moreover, our results indicate that a lowering of the mass density of an a-ZTO sample leads to a rising of the conduction band edge. (C) 2013 Elsevier B.V. All rights reserved.