Carbon monoxide reduces aqueous hydrochloric acid solutions of RhCl3,3H2O under mild conditions to the dichlorodicarbonylrhodate(I) species, [Rh(CO)2Cl2]–, in an autocatalytic reaction. The kinetics indicate that reduction occurs via direct reaction of carbon monoxide with [RhCl5(OH)]2– and possibly [RhCl5(H2O)]2–, followed by a more efficient sequence involving formation of mixed valence RhIII⋯ RhI bridged intermediate. The carbon monoxide is effectively being activated by co-ordination to rhodium, and iron(III) has been catalytically reduced using the carbon monoxide–dichlorodicarbonylrhodate(I) system. At room temperature carbon monoxide rapidly converts fresh aqueous solutions of RhCl3,3H2O to the [RhCl2(H2O)4]+ species via a similar bridge mechanism involving small amounts of chlororhodate(I) species.