The regenerative ‘redox’ mechanism is the most commonly accepted mechanism for the WGSR. It involves a regenerative change in the oxidation state of the catalytic metal. In this mechanism, H2O is activated first by the abstraction of H from water followed by dissociation or disproportionation of the resulting OH to afford atomic O. The CO is then oxidized by the atomic O forming CO2 which returns the catalytic surface back to its pre-reaction state. Alternatively, CO may be directly oxidized by the OH to form a carboxyl intermediate, followed by the dissociation or disproportionation of the carboxyl. Finally H is recombined to H2 and CO2 and H2 are desorbed from the metal. The principal difference in these mechanisms is the formation of CO2. The redox mechanism generates CO2 by reaction with adsorbed oxygen, while the associative mechanism forms CO2 via the dissociation of an intermediate. The mechanism of decarboxylation is debated; it may involve β-hydride elimination, or it may require the action of an external base.