Vanadium-niobium oxide catalysts containing variable amounts of vanadium and prepared by coprecipitation were investigated using 51V solid-state NMR. It was found that, in samples containing less than about 8 mol % V2O5, vanadium occurs in three different oxygen environments: in isolated tetrahedra, in comer-sharing tetrahedra (pyrovanadate), and in distorted octahedra. The number of octahedral sites increased as the amount of vanadium in the bulk of the catalyst was increased. The compound 8-(Nb,V)205 was found to occur in samples containing more than 7 mol % V2O5. In this compound, vanadium occurs not only in isolated tetrahedral sites but also in comer-sharing tetrahedral sites. For a catalyst having the niobia in the crystalline T form, it was found that an increase in the bulk concentration of vanadium caused the number of neighboring sites occupied by vanadium to increase and the vanadium partially to replace niobium in the T-niobia structure.