The [FeFe]-hydrogenases ([FeFe] H(2)ases) catalyze reversible H-2 activation at the H-cluster, which is composed of a [4Fe-4S](H) subsite linked by a cysteine thiolate to a bridged, organometallic [2Fe-25] ([2Fe](H)) subsite. Profoundly different geometric models of the H-cluster redox states that orchestrate the electron/proton transfer steps of H-2 bond activation have been proposed. We have examined this question in the [FeFe] H(2)ase I from Clostridium acetobutylicum (CaI) by Fourier-transform infrared (FTIR) spectroscopy with temperature annealing and H/D isotope exchange to identify the relevant redox states and define catalytic transitions. One-electron reduction of H-ox led to formation of HredH+ ([4Fe4S](H)(2+)-Fe-I-Fe-I) and H-red' ([4Fe-4S]H1+-Fe-II-Fe-I), with both states characterized by low frequency mu-CO IR modes consistent with a fully bridged [2Fe](H). Similar mu-CO IR modes were also identified for HredH+ of the [FeFe] H-2 ase from Chlamydomonas reinhardtii (CrHydA1). The CaI proton-transfer variant C298S showed enrichment of an H/D isotope-sensitive mu-CO mode, a component of the hydride bound H-cluster IR signal, H-hyd. Equilibrating CaI with increasing amounts of NaDT, and probed at cryogenic temperatures, showed HredH+ was converted to H-hyd. Over an increasing temperature range from 10 to 260 K catalytic turnover led to loss of H-hyd and appearance of H-ox, consistent with enzymatic turnover and H-2 formation. The results show for Cal that the mu-CO of [2Fe](H) remains bridging for all of the "H-red" states and that HredH+ is on pathway to H-hyd and H-2 evolution in the catalytic mechanism. These results provide a blueprint for designing small molecule catalytic analogs.