Pulsed ENDOR and HYSCORE measurements were carried out to characterize the active site of the oxygen-tolerant NAD(+)-reducing hydrogenase of Ralstonia eutropha. The catalytically active Ni-a-C state exhibits a bridging hydride between iron and nickel in the active site, which is photodissociated upon illumination. Its hyperfine coupling is comparable to that of standard hydrogenases. In addition, a histidine residue could be identified, which shows hyperfine and nuclear quadrupole parameters in significant variance from comparable histidine residues that are conserved in standard [NiFe] hydrogenases, and might be related to the O-2 tolerance of the enzyme.