Pretreatment of Cs2H2PVMo11O40 catalyst with methacrylic acid (MAA) induces better performance in the oxidation of methacrolein. Pulsed and continuous experiments revealed higher conversion of reagents and increased selectivity to MAA for the pretreated catalyst. XRD showed swelling of the crystal lattice, while FTIR confirmed removal of the MO (x) (M = V or Mo) entities from the Keggin units at 80-100 A degrees C for the MAA-pretreated samples.