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Despite numerous studies on controlled periodic illumination to improve the quantum yield of photocatalytic reactions, debates still exist on the nature of the enhancement effect. Here, we report the promotion of HCOOH dehydrogenation over TiO2-supported noble metal photocatalysts using periodic illumination. Intermittent illumination at a moderate frequency of approximately 7 Hz and 10% duty cycle improved the quantum yield of H2production by more than 2-fold, regardless of the type of metal on TiO2. In conjunction with photoelectrochemical characterisation techniques, we suggest that the promotion mechanism in periodic illumination involves enhancement of the electron charge transfer kinetics for more efficient electron migration to the metal nanoparticles rather than light intensity effects. This explanation is supported by the linear correlation between the quantum yield and photocurrent enhancement factors as functions of duty cycles, which seems to be generalisable among different photocatalysts under periodic illumination. Kinetic isotope effect (KIE) studies suggest the Pt–H cleavage to be a rate-limiting step for H2production under both continuous and periodic illumination.