The mechanism of photochemical formic acid dehydrogenation catalyzed by [Cp*Ir­(bpy)­(Cl)]+ (1, bpy = 2,2′-bipyridine) and [Cp*Ir­(bpy-OMe)­(Cl)]+ (1-OMe, bpy-OMe = 4,4′-dimethoxy-2,2′-bipyridine) is examined. The catalysts operate with good turnover frequency (TOF) across an unusually wide pH range. Above pH 7, the evolved gas is >95% pure H2 (along with traces of CO2 but no detectable CO). Light-triggered H2 release from a metal hydride intermediate is found to be the turnover-limiting step, based on the observed first-order dependence on catalyst concentration, saturation behavior in formate concentration, and direct in situ observation of a metal hydride resting state during turnover. Deactivation pathways are identified, including ligand loss and aggregate formation, precipitation of insoluble forms of the catalyst, and deprotonation of the iridium hydride intermediate. Guided by mechanistic insights, improved catalytic activity (initial TOF exceeding 50 h–1), stability (>500 turnovers at nearly 5 atm), and selectivity (>95% H2 gas) are achieved.