The terminal step in methane generation by the archaeon Methanobacterium thermoautotrophicum is catalyzed by the enzyme S-methyl coenzyme M reductase (methylreductase). This enzyme contains a Ni(II) tetrapyrrole cofactor, F430, at the active site in the resting state. A Ni(I) state (NiIF430) has been proposed as the active form of the cofactor. Nickel isobacteriochlorins have been used to model F430. We have investigated both NiIF430 and NiIOEiBC using CW and pulsed EPR and ENDOR spectroscopy at X-band and Q-band microwave frequencies. In agreement with a previous X-band EPR and ESEEM study, at Q-band, the g tensor of NiIF430 appears axial and 1,2H ENDOR of NiIF430 in H2O versus D2O solvent shows no evidence for strongly coupled, solvent-exchangeable hydrogens, and this indicates that there is no water axially coordinated to Ni(I) in contrast to the Ni(II) resting state. Both NiIF430 and NiIOEiBC give 14N ENDOR signals arising from the four pyrrole nitrogen ligands to Ni(I). Previous EXAFS studies of NiIO...