The dynamic behavior of infrared (IR)-observable species during the methylene blue [i.e., MB, (CH3)2N(C6H3)NS+(C6H3)N(CH3)2Cl-] photocatalytic degradation (PCD) on TiO2 has been investigated at 30 °C. Exposure of MB/TiO2 to UV illumination led to the scission of the N−CH3 bond followed by breaking of the C−H and CN bonds in the MB central aromatic ring and the side aromatic rings, indicating demethylation as the first step of the MB PCD. The bond breaking in the MB molecule and subsequent reactions produced charge-containing intermediates (i.e., carboxylate (RCOO-) and R−NH3+), slowing down the conversion of MB to CO2, H2O, NH4+, and SO42-. Probing the MB PCD by adsorbed ethanol revealed that the demethylation step was initiated by the OH/OD radical (·OH/·OD) and the breaking of CN and CS−C in the MB central aromatic ring by H+/electron transfer. In situ IR coupled with the use of ethanol as a probe molecule provides an excellent method for investigating the PCD mechanism.