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Background: The photocatalytic activity of SnO2-TiO2 nanocomposites was successfully assessed after synthesis by Sol-Gel method, deposition on porous silicon material and annealing at 400, 600 and 800oC temperatures, with surface grain size in the range between 5 and 12 nm. The photocatalyst was characterized by X-ray diffraction (XRD), high resolution scanning electron microscopy (SEM), thermogravimetric analysis (TGA), derivative thermogravimetry (DTG) and mass spectroscopy (MS). The photocatalytic assessment suggests that this SnO2-TiO2 photocatalyst presents important photocatalytic efficiency to methyl blue (MB) at low annealing temperature. Methods: In this work, sol-gel method is used to prepare SnO2-TiO2 nanocomposites; which were deposited on porous silicon, annealed at different temperatures and investigated to understand their structural, morphological, optical and physical properties. Their photocatalytic activity was evaluated by using the degradation of MB under irradiation with UV light. Results: The intensity of the characteristic absorption band of methyl blue at 655 nm decreased significantly with the increasing irradiation time. Meanwhile, a change in color of the solution occurred; turning from blue to colorless after 20 min of irradiation, and thus indicating the gradual decomposition of methyl blue molecules during UV light irradiation in the presence of the prepared SnO2-TiO2. As expected, no degradation of the SnO2-TiO2 nanocomposites occurred under control experimental conditions. The MB degradation efficiency was reported by C/C0 quantity; where C and C0 correspond respectively to its concentration at time t and initial concentration. In the presence of photocatalysts, it can be clearly deduced that after irradiation for 20 min, the C/C0 of the MB value was about 0% with the prepared and untreated nanocomposites of SnO2-TiO2 and remains constant when using a SnO2-TiO2 nanocomposite treated at 800°C. Conclusion: This research has successfully synthesized the SnO2-TiO2 nanocomposite photocatalysts by Sol-Gel process and deposited by spin–coating technique on porous silicon substrates. Besides, all structural, optical and catalytic properties were studied and CO related. The obtained material was annealed at three different temperatures 400°C, 600°C and 800°C. It is denoted that its grain size increases from about 5 nm to 12 nm with the annealing temperature. The photocatalytic effect has been tested on the MB solution, which demonstrates that the nanometric grain size enhances the adsorption properties and achieves a good photocatalytic performance at a low temperature.