VOx monolayer on TiO2 catalysts have been prepared by grafting and subsequently promoted with tungsten, tin, and potassium by impregnation. The samples have been characterized through N-2 physisorption at 77 K, PXRD, Raman spectroscopy, H-2-TPR, and 1H Solid State NMR spectroscopy with NH3 as probe molecule. Catalytic measurements have been performed in an isothermal fixed bed reactor at 350 degrees C with 1.5 vol.% o- xylene in air. Space velocities were varied to get selectivity- conversion-diagrams for CO2 and maleic anhydride as side products as well as phtalic anhydride, toluyl aldehyde and phthalide as value products. The selectivity-conversion diagrams of the unpromoted sample clearly indicate a direct pathway from o-xylene and/or toluylaldehyde towards phthalic anhydride. Tungsten, tin and potassium promoted samples were compared with unpromoted samples. It was found that optimum accessibility and acidity of the promoted VOx monolayer can explain the observed promoter effects. Tungsten as promoter decreases MA selectivities and increases PA selectivities at an optimum loading of 0.12 wt.% WO3. Tin was detrimental at all loadings because acidities seem to be too high causing too strong adsorption and condensation reactions of o-xylene. With potassium as promoter lowest acidities and highest V accessibilities were reached which correlates with the increase of activity and PA selectivity. In contrast to potassium, tungsten and tin move significantly into the TiO2 support during calcination which was proven by XPS measurements on sputtered probes. This might explain why potassium acts more efficiently as promoter already at low promoter levels.