The decarbonylation of methoxy(siloxy)carbene (CH3OCOSiH3) has been investigated by means of ab initio molecular orbital theory and hybrid density functional theory calculations, in conjunction with the quantum theory of atoms in molecules. Pathways involving intramolecular front-side nucleophilic attack by the methoxy oxygen at silicon and by the siloxy oxygen at the methyl carbon have been examined. The pathway involving intramolecular front-side nucleophilic attack by the methoxy oxygen at silicon has been found to be concerted (one step, no intermediates) and synchronous (bond formation and dissociation occur at the same rate). In contrast, although the pathway involving intramolecular front-side nucleophilic attack by the siloxy oxygen at the methyl carbon appears to be concerted, it is asynchronous, with transition-state features that resemble a tight ion-pair intermediate comprising a methyl cation and a siloxycarbonyl anion. Pathways for decarbonylation involving the intermediacy of methyl silyl formate, as well as radicals, have also been investigated. The most energetically viable pathway for decarbonylation appears to be that involving intramolecular front-side nucleophilic attack by the methoxy oxygen at silicon.