Theoretical studies of the gas-phase elimination kinetics of title compounds were performed by using "ab initio" methods at MP2/6-31G, MP2/6-31G(d,p) and ONIOM [MP2/6-31G (d,p) //MP2/6-31G]. Ethyl Oxamate and ethyl oxanilate undergo a rapid decarbonylation to give the corresponding carbamates. These intermediates proceed to a parallel decomposition to give the corresponding unstable carbamic acid and ethylene through sixmembered cyclic transition state (path 1) and isocyanate and ethanol through a four-membered cyclic transition state (path 2). Ethyl N,N-dimethyloxamate elimination reaction yields in one step, through a six-membered cyclic transition state, dimethyl oxamic acid and ethylene gas. The calculated bond orders, NBO charges and synchronicity indicate that these reactions are concerted and slightly asynchronous. The estimated kinetic and thermodynamic parameters are in good agreement with the reported experimental values.