The structure and reactivity of intermediates in the photocatalytic cycle of a proton reduction catalyst, [Fe2(bdt)(CO)6] (bdt = benzenedithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe2(bdt)(CO)6]-, a key intermediate in photocatalytic H2 formation, was generated by reaction with one-electron reductants in laser flash-quench experiments and could be observed spectroscopically on the nanoseconds to microseconds time scale. From UV/vis and IR spectroscopy, [Fe2(bdt)(CO)6]- is readily distinguished from the two-electron reduced catalyst [Fe2(bdt)(CO)6]2- that is obtained inevitably in the electrochemical reduction of [Fe2(bdt)(CO)6]. For the disproportionation rate constant of [Fe2(bdt)(CO)6]-, an upper limit on the order of 107 M-1 s-1 was estimated, which precludes a major role of [Fe2(bdt)(CO)6]2- in photoinduced proton reduction cycles. Structurally [Fe2(bdt)(CO)6]- is characterized by a rather asymmetrically distorted geometry with one broken Fe-S bond and six terminal CO ligands. Acids with pKa ≥ 12.7 protonate [Fe2(bdt)(CO)6]- with bimolecular rate constants of 4 × 106, 7 × 106, and 2 × 108 M-1 s-1 (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe2(bdt)(CO)6H] is therefore likely to be an intermediate in photocatalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe2(bdt)(CO)6], with very similar carbonyl stretching frequencies. © 2014 American Chemical Society.