Resonance Raman (RR) spectroscopy was used to identify a methylnickel adduct (nu(Ni-C) = 422 wave numbers) of carbon monoxide dehydrogenase (CODH) from Clostridium thermoaceticum. Formed at a nickel/iron-sulfur cluster on CODH called center A, the methylnickel species is the precursor of the methyl group of acetyl-coenzyme A in an anaerobic pathway of carbon monoxide or carbon dioxide fixation. Rapid kinetic and RR studies demonstrated that methylation of nickel occurs by heterolysis of the methyl-cobalt bond (nu(Co-C) = 429 wave numbers) of a methylated corrinoid/iron-sulfur protein. In combination with the earlier finding of an iron-carbonyl adduct at center A, defection of the methylnickel intermediate establishes a bimetallic mechanism for acetyl-coenzyme A synthesis.