Light-active dinuclear iridium pentahydride complexes catalyze the decomposition of formic acid to generate H2 by irradiation (λ =395 nm) under ambient temperature and base-free conditions. The catalyst activity is sensitive to light producing H2 under light irradiation, but with no reaction being observed in the absence of light or when the light is switched off, thereby demonstrating the clear ON/OFF switching ability of this system. Importantly, the dinuclear structure of the catalyst is sufficiently stable to be maintained under the catalytic conditions employed herein.