For a long time, the photobleaching of methylene blue (MB) in aqueous solution has been used as a method to evaluate the photocatalytic activity of TiO2. However, in the presence or absence of TiO2, the MB photobleaching pratically follows various pathways and strictly depends on experimental conditions: dissolved oxygen amount in solution, pH, illumination wavelength. Some recent works showed that the MB molecules are preferentially photoreduced to their colouress form in the absence of dissolved oxygen and low pH conditions. On the other hand, a rich amount of dissolved oxygen and higher pH conditions promote the MB photooxydation. The illumination wavelengths also play an important role in deciding the photodegradation mechanism of MB.