Hypotheses as to possible ways of enhancing the selectivity of partial n-pentane oxidation with respect to phthalic, maleic, and citraconic anhydrides are suggested based on the mechanistic concepts of the formation of these products. The hypotheses are checked by studying the properties of vanadium-phosphorus oxide (VPO) catalysts upon the introduction of different concentrations of La, Bi, Fe, W, Te, Ti, Zr, or Mo. It is shown by various physicochemical methods (x-ray diffraction, IR spectroscopy, X-ray photoelectron spectroscopy, and pyridine and 2,6-dimethylpyridine adsorption) that the phase composition of the VPO catalyst and the oxidation states of its elements on the surface are not changed in most cases upon the introduction of these additives. The additives mainly affect the number of Lewis acid sites, whereas the Bronsted acidity changes only slightly. The existence of a correlation between the citraconic anhydride selectivity and the total acidity of the modified catalyst surface is confirmed. In agreement with the assumptions as to the process selectivity, it was discovered that the phthalic anhydride selectivity increases with an increase in the number of Lewis acid sites. The observed regularities open up the way to the purposeful synthesis of catalysts with enhanced selectivity with respect to these products.