The diiron hydride [(mu-H)Fe-2(pdt)-(CO)(4)(dppv)](+) ([H-2](+), dppv = cis-1,2-C2H2(PPh2)(2)) is shown to be an effective photocatalyst for the H-2 evolution reaction (HER). These experiments establish the role of hydrides in photocatalysis by biomimetic diiron complexes. Trends in redox potentials suggests that other unsymmetrically substituted diiron hydrides are promising catalysts. Unlike previous catalysts for photo-HER, [H-2](+) functions without sensitizers: irradiation of [H-2] in the presence of triflic acid (HOTf) efficiently affords H-2. Instead of sacrificial electron donors, ferrocenes can be used as recyclable electron donors for the photocatalyzed HER, resulting in 4 turnovers.