Gas-phase oxidation of methacrolein to methacrylic acid was carried out over an acidmodified Cr2O3/SiO2 catalyst.While only total oxidation occurred over bare Cr2O3/SiO2, the acid-modified Cr2O3/SiO2 showed catalytic activity for the formation of methacrylic acid. In particular, H3PW12O40 strong Brønsted acid was the most effective modifier for improving both activity and selectivity. The interface between Cr2O3 and H3PW12O40 particles on SiO2 appears to be responsible for the formation of active sites for the selective formation of methacrylic acid. The strong Brønsted acid would help the activation of methacrolein through rendering it more electrophilic, which is a key step for the formation of methacrylic acid over the present catalyst.