The geometric and electronic structures of the active sites in the oxidized Ni(r)-B state of the [NiFe] hydrogenases from Ralstonia eutropha H16 and Desulfovibrio vulgaris Miyazaki F were investigated in pulsed EPR and ENDOR experiments at two different microwave frequencies (X- and Q-band). Two hyperfine-couplings were clearly resolved in the frozen solution spectra arising from the beta-protons of the nickel-coordinating cysteine residues Cys549 and Cys586 from the Desulfovibrio vulgaris and Ralstonia eutropha hydrogenase, respectively. ESEEM spectroscopic experiments reveal the presence of a histidine in the second coordination sphere of the Ni. The spectroscopic data indicate that the electronic structures of the [NiFe] centers in both hydrogenases are identical in the Ni(r)-B state. However, additional spin couplings of the active site to further paramagnetic centers were identified for the Ralstonia eutropha hydrogenase. The respective couplings could be clearly resolved and simulated. The results from this study are discussed in view of the exceptional O(2)-tolerance of the Ralstonia hydrogenase.