V/Nb mixed oxides were prepared, characterized and tested as catalysts for the oxidehydrogenation of propane to propylene. The reactivity tests were carried out under co-feed and under redox-decoupling conditions, with alternate-feeds of propane and air (cyclic operation). The comparison between co-feed and alternate-feed operations evidenced that in the latter case a higher selectivity to propylene is achieved at low propane conversion. This was attributed to a reaction mechanism mainly involving propane dehydrogenation rather than oxidehydrogenation, occurring on the reduced catalysts. The selectivity to propylene, however, under both operating conditions decreased with increasing propane conversion. During reaction under redox-decoupling conditions VNbO5 decomposed yielding vanadium oxide and V/Nb mixed oxides having V/Nb ratio lower than 1. The former compound played the major role in the reaction cycle.