The conversion of an unactivated 2-alkylaziridine to the corresponding oxazolidinone generally requires a very high pressure of carbon dioxide, a high temperature, an expensive catalyst, and/or a long reaction time. Here, a new, high yield (over 95%), and highly regioselective (over 95%) conversion of an unactivated aziridine to an oxazolidinone is reported. This reaction is easy to perform because it requires a low pressure of carbon dioxide, low temperature, no co-solvent, and the catalyst is the salt ammonium iodide.