Differently prepared vanadyl pyrophosphate and potassium-doped vanadia catalysts have been studied during the selective oxidation of toluene and p-methoxy toluene to the corresponding benzaldehydes using a portfolio of various in situ-methods (EPR, UV-VIS-DRS, XRD and XPS). The poor catalytic performance of (VO)(2)P2O7 is mainly due to strong product adsorption which is favoured by surface OH-groups formed under reaction conditions. This process can be partially suppressed by adding competitive adsorbates to the feed, in K-V2O5 catalysts, a potassium vanadate phase is formed on the surface in which about 25% of the vanadium ions are tetravalent. This leads to lower acidity and oxidation potential of the surface and, thus, to improved benzaldehyde selectivities.