The interaction of ammonia with (VO)₂P₂O₇ prepared by calcination of the precursor compound VOHPO₄· 0.5H₂O under nitrogen has been studied using temperature-programmed desorption of ammonia (TPDA), temperature-programmed reaction spectroscopy (TPRS), and IR and EPR spectroscopy. Mass-spectrometric detection was applied to observe possible ammonia decomposition or oxidation products. The investigation revealed that ammonia is not only adsorbed on but also reacts with (VO)₂P₂O₇ in a redox process generating nitrogen, water and an amorphous V^III-containing compound, the concentration of which could be directly determined by potentiometric titration. The high amount of V^III found pointed towards reduction of V^IV not only on the surface but also in deeper layers of the bulk. This was also confirmed by EPR spectroscopy. Furthermore, this reaction results in a change of the Brønsted and Lewis acidity observed by IR spectroscopy. The concentration of the Brønsted-acid OH groups was strongly enhanced by hydrolysis of P—O—P and/or V—O—P links by water formed during the redox reaction. The increased concentration of Lewis sites was caused by the removal of oxygen from surface vanadyl groups, probably creating additional coordinatively unsaturated sites. The influence of the observed redox reaction on the characterization of the acidity and the formation of VPO catalysts in the ammoxidation reaction are discussed.