Triethylphosphate impregnation of 2.8 wt% V/SiO2 and subsequent controlled calcination produced phosphorus-modified supported vanadium catalysts. Phosphorus modification enhanced the yield of maleic anhydride in the partial oxidation of butane. Varying the phosphorus to vanadium atomic ratio from 0 to 2.8 increased the selectivity to maleic anhydride from 0 to approximately 48%. The selectivity was nearly constant up to 20% butane conversion and for different O-2/C4H10 ratios. The Raman spectra of the phosphorus-modified samples had bands at 1040 and 930 cm(-1), and broad unresolved bands between 580 and 540 cm(-1). It was concluded that the active phases in these samples were alpha-VOPO4.