Pure Sb0.9V0.9O4 and various preparations with excess of either vanadia or antimony oxide, including mechanical mixtures, have been investigated for propane ammoxidation to acrylonitrile. The catalysts were characterized before and after use in catalysis by various methods, including electron microscopy, infrared spectroscopy and XPS. The catalytic data show that preparations with approximate to SbVO4 and alpha-Sb2O4, compared with the single phases, are more selective to acrylonitrile formation on the condition that the excess antimony oxide is present while synthesising the approximate to SbVO, phase. Considering the catalytic data together with the results from the characterisations, various possibilities are discussed to explain the role of excess alpha-Sb2O4 in propane ammoxidation. Possibilities that can be excluded on rational grounds are catalysis on two phases, or, at grain boundaries, an influence on the morphology of approximate to SbVO4 from alpha-Sb2O4, the formation of alpha-Sb2O4 containing vanadium, defect formation, creation of active sites by the spillover of oxygen, and formation of VSb2O5. Instead, the observed synergy effect is due to the formation of approximate to SbVO4 enriched with antimony at the surface, creating isolation to a suitable level of the V-centres. The explanation is consistent with several observations including catalytic data for a series of vanadium compounds with different vanadium content, showing that structural isolation of the vanadium is necessary for obtaining high selectivity to acrylonitrile.