We have been developing the chemistry of electron-rich aluminium ('aluminyl') compoundsstabilized by NON supporting ligands (4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9dimethylxanthene; Figure ). 51, Such systems have been shown to be capable of the cleavage of C-H, 51 C-C, and C-O bonds in arene substrates, and to allow access to reactive terminal imide species which can cleave the CO bond in carbon monoxide. Systems such as K2[(NON)Al]2 ([1-K]2) have also shown strongly reducing capabilities towards oxygen-containing substrates, and here we report on the reactivity of aluminyl compounds towards CO. We show that the homologation to give the known [C4O4] 4-fragment occurs with both mono-and dinuclear systems, a finding rationalized computationally through a rate-determining C=C bond forming dimerization process. The initial interaction of the CO molecule at aluminium is unusual, in that it behaves primarily as a Z-type ligand, reflecting the highly electron-rich nature of the aluminyl reagent. Subsequent formation of [C6O6] 4-from [C4O4] 4-shows a solution-phase CO homologation process that brings about chain branching via complete C-O bond cleavage. In addition, comparison of the linear [C4O4] system with the (unprecedented) [C4O4] 6-congener formed under more reducing conditions, models the conversion of C-O bonds to C-C bonds in the presence of additional reductant. -via CO homologation. The reaction of potassium aluminyl compound [1-K]2 51 with CO (ca. 1.5 atm) in benzene over 16 h at ca. 35 ºC (followed by recrystallization from toluene) leads to the isolation of the orange crystalline product