dGas phase methanol partial oxidation was carried out on a highly ordered vanadium substituted MCM-41 and an acid site related active site is proposed for this reaction. All synthesized samples showed similar values for the methanol percent conversion and the formaldehyde selectivity independent of the vanadium content. The turnover frequency of methanol and formaldehyde was also similar when it was normalized by the accessible active site concentration measured by the oxygen titration at reaction temperature. The active site concentration was found to be correlated with the acid site concentration of V-MCM-41. The main active site is proposed to be a weak Lewis acid site, which has an isolated tetrahedral coordination of vanadium with surrounding oxygen anions. V-MCM-41 is a stable catalyst against gas phase methanol oxidation at 350 degrees C. The saturation value of vanadium concentration in this catalytic system was estimated to be 0.41 wt.%, and produces a very stable isolated tetrahedral coordination V site.