The role of Nb2O5 and gamma-Al2O3 oxide supports on the ammoxidation of propane on supported mixed Sb-V oxide at different Sb + V surface coverages is studied. Sb and V oxide species on alumina and on niobia support show different structural features that reflect in different performance during the aminoxidation of propane to acrylonitrile. Niobia-supported catalysts are much more selective to acrylonitrile than alumina-supported ones. Alumina interacts weakly with the supported oxides while niobia forms new phases through solid state reactions with the supported oxides during catalytic operation that must account for its higher selectivity values towards acrylonitrile and higher specific rate of acrylonitrile formation per vanadium site.