A microfluidic device was applied to the photocatalytic degradation of methylene blue as a model pollutant. Titanium dioxide nanoparticles (TiO2-P25) and a synthesized composite TiO2-graphene catalyst were immobilized on the inner walls of a borosilicate glass microfluidic chip. The deposition evolution of the nanoparticles was evaluated by monitoring the optical profile of the system. It was found that a higher initial reaction rate was obtained in the microreactor containing composite catalyst (TiO2-GR) on the inner walls, but both systems (TiO2 and TiO2-GR) achieved similar reaction rates when the steady-state was reached. Decolorization rate of methylene blue in our microfluidic chips was found to be approximately one order of magnitude higher than equivalent macroscopic systems reported in the literature at similar experimental conditions. Additionally, computational simulations were performed to investigate the physics involved in these processes. The model was experimentally validated for further scale-out studies. (C) 2016 American Institute of Chemical Engineers