Syntheses in the Al-Sb-V-O system, which is a candidate catalyst system for propane ammoxidation to acrylonitrile, were characterised by X-ray powder diffraction, electron diffraction, energy dispersive X-ray microanalysis, high-resolution electron microscopy, infrared spectroscopy, and catalytic measurements. The objective was to study phase formation as well as to identify the phase, or phases, which are active and selective for propane ammoxidation and to conclude about the catalytic role of the various types of metal ion centres. Calcination in air at 680 degrees C of mixtures of Al(OH)(3), Sb2O3, and V2O5 showed the formation of alpha-Sb2O4, V2O5, AlVO4, and two rutile-related phases Sb0.9V0.9O4 and Al1-xSbVxO4, 0 < x < 0.5. The latter presents a trirutile-type superlattice as shown by electron diffraction and high-resolution transmission electron microscopy. Formation of Al1-xSbVxO4 was not observed starting from the phase composition, but required an excess of alumina (> 60 at.% Al). Without alumina in excess, Sb0.9V0.9O4 is formed. Activity measurements over phases belonging to the Al-Sb-V-O system revealed that Al1-xSbVxO4 is the most selective toward acrylonitrile formation, and it was identified as the active phase in a sample of patent type with Al : Sb : V = 21 : 5 : 1. Rationalisation of activity and selectivity data for V2O5, Sb2O4, Al2O3, Sb0.9V0.9O4, AlVO4, AlSbO4, Sb0.9V0.9O4/alpha-Sb2O4, beta-Sb1.9V0.1O4, and Al1-xSbVxO4 showed that in oxides with vanadium, it is the V-centres which are active and determine the specific activity. To have a catalyst that is selective to acrylonitrile and to avoid combustion, it is necessary to have structurally isolated V-centres which are surrounded by Al and/or Sb.