The oxidative titrations of hydrogenase (Hase) from Desulfovibrio gigas [Barondeau, D. P., Roberts, L. M., & Lindahl, P. A. (1994) J. Am. Chem. Soc. 116, 3442] were simulated using model descriptions of the redox reactions in the enzyme. The data fit best to a model that assumed Hase contains one [Fe3S4]1+/0 cluster, two [Fe4S4]2+/1+ clusters, and a Ni center stable in four redox states (Ni-B, Ni-SI, Ni-C, and Ni-R), each separated by one electron. A model in which Ni-SI, Ni-C, and Ni-R correspond to Nickel(2+) dithiolate, nickel(1+) dithiol, and nickel(2+) dithiol hydride, respectively, is compatible with all established relevant properties of the Ni center. This model and the concept of redox microstates were employed to define electronic states of the enzyme and to reformulate the catalytic mechanism initially proposed by Cammack et al. [Cammack, R., Patil, D. S., Hatchikian, E. C., & Fernandez, V. M. (1987) Biochim. Biophys. Acta 912, 98] into three interconnected catalytic cycles. These cycles differ in the average oxidation level of the Fe4S4 clusters. The cycle with the most reduced clusters appears to operate reversibly (catalyzing both H2 oxidation and H+ reduction), while those with more oxidized clusters function only to oxidize H2. The difference in reversibility is explained by assuming that Ni-R prefers to reduce an [Fe4S4]2+ cluster instead of H+ and that H+ is reduced only when that Fe4S4 cluster is in its reduced state.