ZnO thin films were deposited inside of fused silica tubing by aerosol assisted chemical vapor deposition technique. The films were transparent, uniform, highly adherent and non-light scattering. Photocatalytic activity of internally ZnO coated tubing was evaluated by discoloration of a methyl orange aqueous solution in a batch reactor. Tubing was externally irradiated with UV-A at room temperature. A one dimensional model was proposed to calculate the spatial distribution of the carrier density and the films’ surface charge carrier concentration. This model can explain the influence of the films thickness on the photocatalytic activity. Results showed that the photocatalytic activity largely depends on the film thickness. For external irradiation of the films the optimum thickness was around 60–70 nm, for which the photocatalytic activity was maximum. The photonic efficiency of internally ZnO coated tubular reactors was evaluated as a function of initial colorant concentration, irradiation time and intensity. Furthermore, due to the high activity of the ZnO films, the films were repeatedly exposed to UV-A irradiation cycles, followed by activity measurement.