Cobalt(III) complexes of the type [CoIII(chel)(OH)(H2O)], where chel is a tetradentate conjugated chelating ligand, react in water with CO yielding, in the pH range 4–13, [CoII(chel)]. Formation of [CoIII(chel)(CO2H)(H2O)] through nucleophilic attack of OH– on the intermediate [CoIII(chel)(CO)(OH)] is assumed. The carboxy-derivative decomposes yielding CO2 and [CoI(chel)]– species which, in turn, gives [CoII(chel)] by different reaction paths depending on pH of the solution. The same mechanism operates for the complex with the 2,3,9,10-tetramethyl-1,4,8,11-tetra-azaundeca-1,3,8,10-tetraen-11-ol-1-olato anion(tn) up to pH 8. In more alkaline solution (pH 13) the corresponding carbonyl derivative [CoI(chel)(CO)] can be isolated. With all chel complexes investigated the reaction in alkaline solution appears to be autocatalytic. Mono- and di-organometallic derivatives can be prepared by the same reaction in the presence of organic halides.