Protein film voltammetry studies of the [NiFeSe]-hydrogenase from Desulfomicrobium baculatum show it to be a highly efficient H2 cycling catalyst. In the presence of 100% H2, the ratio of H2 production to H2 oxidation activity is higher than for any conventional [NiFe]-hydrogenases (lacking a selenocysteine ligand) that have been investigated to date. Although traces of O2 (<< 1%) rapidly and completely remove H2 oxidation activity, the enzyme sustains partial activity for H2 production even in the presence of 1% O2 in the atmosphere. That H2 production should be partly allowed, whereas H2 oxidation is not, is explained because the inactive product of O2 attack is reductively reactivated very rapidly, but this requires a potential that is almost as negative as the thermodynamic potential for the 2H(+)/H2 couple. The study provides further encouragement and clues regarding the feasibility of microbial/enzymatic H2 production free from restrictions of anaerobicity.