An extended study of a novel visible-light-driven water reduction system containing an iridium photosensitizer, an in situ iron(0) phosphine water reduction catalyst (WRC), and triethylamine as sacrificial reductant is described. The influences of solvent composition, ligand, ligand-to-metal ratio, and pH were studied. The use of monodentate phosphine ligands led to improved activity of the WRC. By applying a WRC generated in situ from Fe3(CO)12 and tris[3,5-bis(trifluoromethyl)phenyl]phosphine (P[C6H3(CF3)2]3, Fe3(CO)12/PR3=1:1.5), a catalyst turnover number of more than 1500 was obtained, which constitutes the highest activity reported for any Fe WRC. The maximum incident photon to hydrogen efficiency obtained was 13.4 % (440 nm). It is demonstrated that the evolved H2 flow (0.23 mmol H2 h−1 mg−1 Fe3(CO)12) is sufficient to be used in polymer electrolyte membrane fuel cells, which generate electricity directly from water with visible light. Mechanistic studies by NMR spectroscopy, in situ IR spectroscopy, and DFT calculations allow for an improved understanding of the mechanism. With respect to the Fe WRC, the complex [HNEt3]+[HFe3(CO)11]− was identified as the key intermediate during the catalytic cycle, which led to light-driven hydrogen generation from water.