The Mo10−xVxOy solid-solution systems (0≤x≤10) were studied by electron paramagnetic resonance spectroscopy. The results show the existence of paramagnetic vanadyl VO2+ species, whose concentration becomes maximal for Mo5V5Oy. A quantitative analysis of the [VO2+] concentration as a function of the Mo/V ratio allows it to characterize the prevailing defect chemistry in the Mo10−xVxOy system. In this respect, the semi-conducting properties of Mo10−xVxOy are p-type in an interval of Mo9V1Oy–Mo5V5Oy and switch into n-type because of the conduction electrons in a composition range of Mo5V5Oy–Mo1V9Oy. Highest catalytic activity is obtained when vanadium acts as an acceptor center and oxygen vacancies are formed for reasons of charge compensation. In addition to the surface, and VO2+ centers in the bulk have to be considered too for heterogeneous catalysis.