Photodegradation of a series of dyes (Rhodamine B, Orange II, Sulfu-rhodamine B, Fluorescein, Alizarin red, Squarylium cyanine, and Eosin) in the presence of TiO2 particles under air-equilibrated controlled conditions and visible light illumination led to the formation of hydrogen peroxide. Combined with chemical oxygen demand (COD) measurements and in situ adsorption of added H2O2, the results reveal that H2O2 is a perfidious intermediate species in the process of dye photodegradation. The formation rate of H2O2 depends on the rate of dye degradation. It can also be decomposed by TiO2 particles under visible light irradiation because of a TiO2/H2O2 surface complex. H2O2 was detected because its formation rate was greater than its decomposition rate; failing this, H2O2 would not be observed. Its decomposition rate depends on the amount of substrates or intermediates formed during dye degradation and adsorbed on the surface of the TiO2 particles. The greater the degradation rate of the dye or the greater the quantity of substrates or intermediates adsorbed on the TiO2 surface, the less is the depletion of H2O2 with the consequence that a large quantity of H2O2 accumulates in the solution bulk permitting its facile detection. Also, pH appears to play a beneficial role in the observation of H2O2 formed.