NbOPO4 and alpha-VOPO4 can easily form a solid solution such as V1-xNbxOPO4, because NbOPO4 is isostructural to alpha-VOPO4 with a tetragonal structure. The VNbPO catalysts prepared by the reduction of V1-xNbxOPO4 with benzyl alcohol form compounds of the type (V1-xNbxO)(2)(P2O7)(1-x)(PO4)(2x). The catalyst keeps the oxidizing ability of (VO)(2)P2O7 and gives strong Lewis acid sites. As a result, VNbPO catalysts have high catalytic activity in the oxidation of butane to maleic anhydride.