Catalytic oxidation of methacrolein (MAL) to methacrylic acid (MAA) over SiO2-supported H4PMo11VO40 with different H4PMo11VO40 loadings was investigated. H4PMo11VO40/SiO2 showed high activity in comparison with unsupported H4PMo11VO40, and 3.3 mol% H4PMo11VO40/SiO2 (50 wt.% H4PMo11VO40) had the highest activity, which was five-times larger than that of unsupported H4PMo11VO40 due to high dispersion of H4PMo11VO40 on SiO2, as determined by temperature-programmed desorption of benzonitrile. On the other hand, the supported catalysts were less selective toward the formation of MAA. From X-ray diffraction and Raman spectroscopy, it was determined that H4PMo11VO40 decomposed to form MoO3 on SiO2 during the catalytic reaction. Since SiO2-supported MoO3 and unsupported MoO3 had only very low selectivity toward the formation of MAA in the oxidation of MAL, it was concluded that the formation of MoO3 caused the decrease in the catalytic performance of the supported catalysts. (C) 2010 Elsevier Inc. All rights reserved.