Density functional theory (DFT) and second order Moller-Plesset perturbation (MP2) calculations, employing the 6-311++G(d,p) basis set, were carried out on alkyl-substituted aziridines to explore the reaction mechanisms and regioselectivity associated with their ring-opening conversions to oxazolidinones, in the presence of carbon dioxide. Computational results, employing the self-consistent reaction field polarizable continuum model (SCRF(PCM/Bader)), indicated that the conversions proceed with thermodynamic ease in THF solvent at room temperature. It is proposed that the N-alkylaziridine promotes ring opening through a S(N)2 attack of the iodide ion, of catalytic lithium iodide, on the preformed complex. The oxazolidinone regioisomer ratio is highly sensitive to aziridine ring-carbon substitution. Therein, monophenyl substitutions show preference to opening more highly substituted carbon-nitrogen bonds, providing rationale as to why experimental works result in an exclusive oxazolidinone regioisomer product.