The aqueous solution reactions of heterometallic cube derivatives of [Mo 3 S 4 (H 2 O) 9 ] 4+ , written here as [Mo 3 MS 4 (H 2 O) 10 ] 4+ (tetrahedral M = Fe, Co, Ni, Pd, Cu) with carbon monoxide are considered. Thermodynamically favourable reactions complete in 330 min are detected with M = Co (Group 9), and Ni and Pd (Group 10) containing cubes having 15 and 16 metal electrons, respectively, but no corresponding reactions are observed with M = Fe, Cu cubes having 14 and 17 metal electrons, respectively. A similar reaction is observed with the Se-containing cube [Mo 3 NiSe 4 (H 2 O) 10 ] 4+ , and with the higher oxidation state cube [Mo 3 CuS 4 (H 2 O) 10 ] 5+ (16 electrons) [Mo 3 S 4 (H 2 O) 9 ] 4+ and Cu(CO) + are obtained as final products. On bubbling N, through the latter CO is removed, and approaching quantitative reformation of [Mo 3 CuS 4 (H 2 O) 10 ] 5+ is observed. Greater difficulty is experienced in removing carbon monoxide from the other carbonyl adducts (M = Co, Ni, Pd) using N 2 .