Vanadium-containing catalysts used in the partial oxidation of toluene and substituted toluenes, resp.. to their aldehydes need two different catalyst functions: (i) acidic surface sites (Lewis-acid sites) for reactant chemisorption and (ii) basic sites for an easier desorption of products. Therefore, the simultaneous addition of nonoxidizable pyridine to the feed leads to an increased aldehyde selectivity at similar conversion compared to runs without co-feeding of pyridine. This result is mainly due to a blockade of Bronsted-acid sites of the catalyst surface. Additionally, the desorption of the desired products could be sped up by an increased basicity of the catalyst surface. Beside catalyst properties. also different substituent effects play an important role for chemisorption. For example, the addition of pyridine to a p-chlorotoluene-containing reaction feed does not enhance aldehyde selectivity, otherwise, pyridine addition to p-methoxytoluene-containing feed results in drastically increased aldehyde selectivity.