A novel reactor designed to study the effects of continuous or controlled periodic illumination (CPI) on photocatalytic reactions was built and tested. The reactor uses immobilized films of TiO2 on the circular face of a disk. Rotating disk hydrodynamics provide uniform access to the catalyst surface. These coated disks rotate in a closed cell filled with the reagents at angular velocities ranging from 20 to 100 revolutions per minute (rev/min). A bank of black lamps provides uniform UV illumination to the disk surface. A mechanical shutter is used to provide the periodic illumination. This shutter can provide light or dark times as short as 100 ms and as long as minutes. To evaluate the performance of this reactor, the oxidation of formate ion (HCOO-) to CO2 and H2O was studied at various light intensities and a single light and dark time. As the light intensity was increased from 0.05 to 5.5 mW/cm(2) the photoefficiency for continuous illumination experiments decreased from 80% to 5%. At a light time of 0.6 s and a dark time of 2.0 s and a light intensity of 5.5 mW/cm(2), the photoefficiency increased from 5% during the continuous illumination experiments to 20% with CPI. However, at low light; intensities (I < 0.5 mW/cm(2)), CPI did not effect the photoefficiency. Analysis of the results indicates that the reactor is oxygen diffusion-limited at light intensities above 0.5 mW/cm(2) when air is used as the oxidant. At intensities below 0.3 mW/cm(2), the reaction is photon limited and we are able to study the kinetics of the reaction. At light intensities between 0.3 and 0.5 mW/cm(2), the reaction is controlled by both surface kinetics and diffusion limitations.