This study examined the effect of thickness of photocatalyst film immobilized on a buoyant polypropylene (PP) substrate on the degradation of methyl orange (MO) dye in aqueous solutions using different light sources. The photocatalyst used was titanium dioxide (TiO2) and the immobilized photocatalyst film thickness varied in a range from 6.9 to 34.1 mu m. Both degradation experiments and model fitting analysis for the experimental data were carried out. All the experimental results on MO dye degradation were found to be well fitted by a pseudo-first-order reaction kinetic model. However, the degradation performance and the reaction kinetic model rate constant showed a clear trend of increase with the increase of the photocatalyst film thickness under the visible (Vis) light irradiation, but had no obvious change with the photo catalyst film thickness under the ultraviolet (UV) light irradiation. This phenomenon was attributed to the much smaller active photocatalyst film thickness for the UV light (due to the lesser quantity of photons and hence less thickness through which the light penetrated), as compared to that for the Vis light in the photocatalytic reaction. The study reveals that methods effectively increasing the photocatalyst film thickness on the buoyant substrate in the preparation of the buoyant composite photocatalyst are beneficial for the photocatalytic degradation of MO dye, especially for situations when the system may work mainly with the visible light, such as using the natural sunlight as the light source.