We have investigated how the phosphorus selectivity-activity moderator interacts with active sites on industrial vanadyl pyrophosphate catalysts used for n-butane oxidation to give maleic anhydride. Combining phosphoric acid doping experiments with catalytic performance tests and several spectroscopic techniques, active sites could be identified. It is shown that the sites consist of coordinatively unsaturated Lewis acid V⁴⁺ centers and Brønsted acid P─OH groups that are connected by V─O─P bridging bonds. Phosphorus blocks these sites but does not alter their chemical nature. Catalytic performance tests correlate with spectroscopic results, demonstrating a direct relationship between the proposed active site structure and the catalytic behavior. It is suggested that the active sites exist in the previously identified amorphous surface layer on the crystalline vanadyl pyrophosphate probably formed by VOPO₄ species as proposed in the literature. The data show that only about 11% of the catalyst surface is catalytically active, making earlier active site suggestions based on crystallographic considerations of vanadyl pyrophosphate unlikely. At high phosphorus loadings, the β-VO(PO₃)₂ phase forms which has a known increased intrinsic activity but lower selectivity to maleic anhydride. The findings contribute to a more comprehensive understanding of the maleic anhydride catalyst functionality.