In the present work, photocatalytic degradation of methylene blue has been carried out using naïve titanium dioxide, nano-sized titanium dioxide and H2O2 under visible light. The catalysts, naïve and nano-sized titanium dioxide were prepared by the sol–gel method. Characterization of synthesized catalysts has been done by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV–Visible spectroscopy. The rate of degradation of dye was monitored spectrophotometrically by measuring absorbance of dye at regular time intervals. The effect of various parameters such as pH, concentration of dye, amount of catalyst, amount of H2O2 and light intensity on the rate of reaction has been studied. Various parameters like chemical oxygen demand (COD), conductance, pH, TDS, salinity and dissolved oxygen (DO) for the reaction mixture have also been determined. A tentative mechanism for degradation of dye has been proposed involving ·−OH radical as an oxidant. The participation of ·−OH radical has been confirmed by using 2-propanol (scavenger) as the rate of reaction was drastically reduced in its presence.