The performance of a photocatalytic reactor system with a parallel array of four 6W blacklight blue fluorescent lamps (wavelength: 300-400nm) was investigated, based on the decomposition of formic acid in an aqueous solution. An aqueous solution of formic acid (7.8-22.0 gm(-3)) was recirculated between the photocatalytic reactor and perfectly-mixed flow container. The results show that the UV-light that penetrated through the wall of a glass tube and then passed through the flowing liquid solution accelerated the photocatalytic reaction occurring on the neighboring glass tubes, which greatly contributed to an increase in the reactor activity. This confirms that the arrangement of several light sources in parallel which uses effectively a three-dimensional space can lead to increased reactor activity and an increase in decomposition rates. A significant reduction in the reaction rate due to a film-diffusional resistance in the vicinity of the titanium dioxide film was observed.