VPO catalyst transformations were investigated using in situ laser Raman spectroscopy. During reduction-oxidation step changes, (VO)(2)P2O7 was readily converted to alpha(II)-, delta-VOPO4, and ultimately to beta-VOPO4 in O-2/N-2; these V5+ phases were eliminated in n-butane/N-2. A wet N-2 feed (5-10% H2O in N-2) transformed (VO)(2)P2O7 and alpha(I)-, alpha(II)-, beta-, delta-, gamma-VOPO4 to V2O5 at temperatures above 400 degrees C. The presence of water vapor facilitated the loss of oxygen atoms involved in V-O-P bonding, and separated vanadium oxide and phosphorus oxide species were formed. The isolated vanadium oxide species could be transformed to V2O5; phosphorus species likely diffused from the catalyst lattice in the form of acid phosphates.