In a functional model study of carbon monoxide dehydrogenase (CODH), a homologous series of Ni(II) complexes with a biologically relevant O, N, S ligand set has been synthesized and characterized. In aqueous solution at room temperature, they are active for CO oxidation by methylviologen (= mv(2+)) to produce CO2. The key features of the reaction are pseudo-first-order dependence on catalyst, CO, H2O, and mv(2+), a sigmoidal rate-pH profile with an inflection point at pH 7.6, and the absence of any H-2 as a product, although H-2 is the exclusive product of the related water gas shift reaction. The proposed mechanism, involving decarboxylation of a Ni-COO- intermediate by mv(2+) in the key step, accounts for all these features. As in CODH itself, CO oxidation is inhibited by both CN- and MeI. O-2 is also a competent electron acceptor in this system because reduced mv(+) is air-sensitive.