A new family of vanadium phosphorus oxides (VPO) catalysts has been identified. It consists in a mixture of VPO with niobium phosphate (NbPO). The amorphous NbPO material is introduced during the preparation of the VOHPO4. 0.5H(2)O precursor. It is observed that the VPO-NbPO catalyst is more rapidly activated and gives better performances (n-butane conversion and maleic anhydride selectivity) for mild oxidation of n-butane to maleic anhydride. VPO phases and the NbPO material have been identified in the VPO-NbPO precursor and in the VPO-NbPO catalyst. Nb in VPO crystals and V in NbPO particles have been respectively observed by EDX-STEM. No other VPO crystals than the VOHPO4, 0.5H(2)O precursor, and (VO)(2)P2O7 for the catalyst, have been identified by XRD and P-31 NMR. P-31 NMR by spin echo mapping and P-31 MAS NMR have confirmed an interaction of the VPO precursor with Nb and of the NbPO amorphous material with V, as evidenced by EDX-STEM. This should be the reason for the observed improvement of the catalytic results by a redox effect of niobium on the VPO catalyst modifying the V5+/V4+ balance in a favourable way. (C)2000 Elsevier Science B.V. All rights reserved.