Co₂(CO)₈, when absorbed in the gas phase, in vacuo, on fully dehydrated MgO, forms a variety of carbonyl clusters both neutral and ionic. These clusters disintegrate into monometallic species upon dosing with either CO or NH₃, resulting in Co(CO)^–₄ and Co²⁺(CO)_n(n= 2 or 3) with CO and Co(CO)^–₄ and Co²⁺(NH₃)₆ with NH₃, respectively. The negatively charged species are formed via several routes, including disproportionation and nucleophilic attack by O^2– on the carbonyl groups of different cobalt carbonyl clusters. These effects are attributed to the strongly basic nature of the highly dehydroxylated magnesia used in the present investigation. Diffuse reflectance and e.s.r. spectral studies performed in similar conditions on magnesia confirm all these transformations. The results on a highly dehydrated SiO₂ surface are quite different: Co₂(CO)₈ is mostly adsorbed without appreciable chemical modification, giving two isomers containing linear and bridged CO. Only by removing CO by outgassing at the beam temperature, is the total transformation into Co₄(CO)₁₂ and possibly Co₆(CO)₁₂ observed.