The photodegradation of methyl orange (MO) was investigated in aqueous suspension containing titania nanoparticles with mesostructures(m-TiO2) under UV irradiation. The experimental results show that 98% MO can be mineralized in the 1.0 g l_1 m-TiO2 suspension(pH 2.0) after 45 min illumination. Particular attention was devoted to the identification and the transformation of the fragmentsretaining the chromophoric group. The photodegradation mechanism of the quinonoid MO mainly involves three intermedial processes:demethylation, methylation and hydroxylation. Among those processes, demethylation is more favorable than the hydroxylation, but thehydroxylation results in the largest number of intermediates. The degradation pathway of quinonoid MO under the optimal conditions isalso proposed.