V-P-O catalysts supported on the surface of silica and titania particles were studied and compared with bulk V-P-O. The catalytic performance was tested in the n-butane oxidation reaction to maleic anhydride, and the structure of the equilibrated catalysts was characterised with X-ray absorption spectroscopy (EXAFS) and (low-temperature) ESR spectroscopy. Our results show considerable differences in catalytic performance between VPO/TiO2 on the one hand, and VPO/SiO2 and VPO/bulk on the other hand, the yield to maleic anhydride being comparable for VPO/bulk and VPO/SiO2. The differences in catalytic behaviour are attributed to differences in the local structure around vanadium (EXAFS). Furthermore, different spin exchange interactions between vanadium atoms in the three samples have been observed (ESR). The combination of characterisation methods suggests that the structure of the supported V-P-O phase is amorphous and differs considerably from that of bulk crystalline vanadylpyrophosphate. We therefore propose that the oxidation of n-butane to maleic anhydride takes place over an amorphous surface V-P-O phase. This finding has high relevance for our understanding of the catalytic activity of bulk crystalline V-P-O catalysts as well.