Lithium dimethylamide (monomer and dimer) and several carbonyl complexes proposed as intermediates in its CO insertion reaction have been investigated by means of ab initio calculations (6-31+G//6-31G and MP2/6-31+G*//6-31G). The calculated values of the main geometrical parameters of the dimer are very close to those from solid-state determinations with stable lithium dialkylamides. The dimerization energy is predicted to be -60.7 and -59.9 kcal/mol at 6-31+G//6-31G and MP2/6-31+G*//6-31G levels, respectively. Calculations show a eta(2)-coordinated lithium in the first intermediate derived from lithium dimethylamide monomers and CO and a rather long C-O bond (1.30 Angstrom). This indicates a; significant alkoxycarbene character, rather than the classical carbamoyl structure. A second intermediate arising from a second CO insertion exhibits a planar geometry with relatively short O-Li bonds and the lithium atom coordinated to both oxygens. The calculations predict the double carbonylation to be a thermodynamically favorable process (-37.6 and 27.7 kcal/mol at 6-31+G and MP2/6-31+G* levels, respectively), in contrast to previous reports but in agreement with experimental results. A third intermediate, formally produced by the coupling of the two just described, shows two eta(2)-coordinated lithiums and a planar arrangement of the main heavy atoms : the calculated high stability (-85.8 and -91.2 kcal/mol at 6-31+G//6-31G and MP2/6-31+G*//6-31G levels, respectively), explains the high yields of substituted hydroxymalonamides obtained under special reaction conditions. The properties of the tetramethylurea dianion proposed as the precursor for dimethylformamides were also calculated : the high electron density (-1.113) found for the central carbon atom predicts the facility of this intermediate for producing tetramethylureas.