In the course of developing a process for selective synthesis of terephthaldehyde by the gas-phase oxygen oxidation of p-xylene over the CVD Fe/Mo/DBH catalyst, it was found that a large reactivity gap exists between p-xylene and o-/m-xylene isomer due to the para-selective nature of the catalyst. Reactivity of xylene isomers for oxidation is in the order p-xylene much greater than o-xylene > m-xylene. p-Xylene was preferentially oxidized to terephthaldehyde and p-tolualdehyde from the xylene isomer mixture as well as the commercially available xylene isomer feed containing ethylbenzene, while the rest of the components remained almost intact. By coating the CVD Fe/Mo/DBH catalyst with tetramethyl-orthosilicate, the preferential oxidation of p-xylene over its isomers became even more selective.