We have investigated the reduced, thionine-treated at 20 degrees C, and thionine-treated at 80 degrees C forms of Pyrococcus furiosus [NiFe] hydrogenase using L-edge X-ray absorption spectroscopy. At 20 degrees C, the Ni site is apparently not redox active, since the reduced and 20 degrees C thionine-treated forms exhibit the same spectra. Results of theoretical simulations as well as comparison with the spectra of model compounds indicate the presence of high-spin Ni(II) in these forms. On the basis of a comparison with the spectral features of model nickel complexes, the nickel site in the hydrogenase appears to be 5- or 6-coordinate. The 80 degrees C thionine-treated form has a broader Mi L-edge centered at a higher energy, consistent with a charge distribution of at feast two holes on the nickel and at least one hole significantly delocalized onto the ligand framework.