This study investigated the effect of controlled periodic illumination on the photonic efficiency and temporal decomposition behavior of o-cresol by UV/TiO2 process in a slurry reactor using light emitting diodes as the light source. Based on the same UV light intensity of 6.7 W m(-2), the calculated photonic efficiencies increased from 1.5% to 6.3% with the decreasing duty cycle from 1.0 to 0.1 possibly ascribed to the decreasing electron-hole recombination. Actually, the calculated photonic efficiency under periodic illumination at sufficiently intermittence was inferior to, but approached, those under continuous illumination at equivalent average UV light intensity. The calculated electric energy per order increased from 32 to 37 k W h m(-3)order(-1) for experiments conducted with the increasing duty cycle from 0.1 to 1.0, and the electric energy consumption decreased significantly due to the lower duty cycle and average UV light intensity. The photocatalytic decomposition of o-cresol with the UV-light emitting diodes light could be well modeled by the Langmuir-Hinshelwood kinetic equation.