The reaction mechanism and intermediates of the photocatalytic degradations of alachlor in TiO2 suspensions using narrow-band UV radiation with a peak at 300 nm were investigated. The direct photolysis of alachlor was found to be an insignificant and slow process, but the addition of TiO2 enhanced the reaction rates by about 12 times. Intermediates were identified using liquid chromatography-electrospray ionization-mass spectrometry and MS/MS. A total of 8 major intermediates were identified. The evolution profiles of different intermediates were quantified and the reaction mechanism was proposed accordingly. The major degradation mechanisms of alachlor photocatalysis include dechlorination, dealkylation, hydroxylation, scission of the C-O bond, and N-dealkylation. Both the decay of alachlor and the total organic carbon (TOC) increased slightly with initial pH levels. The TOC analysis revealed the different degradation stages of the reaction, where a lag phase with very low TOC decay was followed by a mild TOC decay and a fast pseudo-first-order TOC decay.