The nature of photon interaction and reaction pH can have significant impacts on semiconductor photocatalysis. This paper describes the effect of pH on the photonic efficiency of photocatalytic reactions in the aqueous phase using TiO2 catalysts. The reactor was irradiated using periodic illumination with UV-LEDs through control of the illumination duty cycle (gamma) through a series of light and dark times (T-on/T-off). Photonic efficiencies for methyl orange degradation were found to be comparable at high gamma irrespective of pH. At lower gamma, pH effects on photonic efficiency were very distinct across acidic, neutral and alkaline pH indicating an effect of complementary parameters. The results suggest photonic efficiency is greatest as illumination time, T-on approaches interfacial electron-transfer characteristic time which is within the range of this study or charge-carrier lifetimes upon extrapolation and also when electrostatic attraction between surface-trapped holes, {(TiOH)-O-IV}(ads) and substrate molecules is strongest. (C) 2014 Elsevier B.V. All rights reserved.