<p>A comparative study on the effect of thickness in the photocatalytic decoloration of methylene blue (MB) dissolved in water was conducted using three semiconductor oxides: zinc oxide (ZnO), tin oxide (SnO₂), and titanium dioxide (TiO₂). These oxides were used in the form of thin films with varying thicknesses of approximately 100, 200, and 300 nm. All films were deposited using the cost-effective dip-coating technique at a moderate processing temperature of 400 °C on inexpensive glass substrates with sol–gel solutions. The structural, morphological, and compositional properties of the films were analyzed using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy, respectively. The degradation of MB was studied by varying the thickness and using three different types of semiconductor oxides through measurements of the optical absorbance of the decolorized liquid by the photocatalytic process. Structural analysis showed that ZnO and SnO₂ films were polycrystalline with preferential growth of (002) and (110) planes, respectively, whereas an amorphous structure was observed for TiO₂ films. Additionally, morphological properties confirmed that the surfaces of all films were composed of nano-grains in round and irregular shapes. Based on all the results reported in this work, it is apparent that the authors believe the degradation efficiency of MB depends on the crystal structure and morphological surface, which in turn depends on the thickness of the film and the type of semiconductor material used as a catalyst. The highest degradation efficiencies, tested with UV–vis radiation for 120 min and an initial concentration of 10 mg/L, were 100, 88, and 76% for ZnO, SnO₂, and TiO₂ films, respectively. These degradation efficiencies were observed in the films with the greatest thicknesses. It was found that as the thickness of the deposited films increases, their physical properties improve, which in turn enhances their efficiency in MB degradation.</p><p></p>