The catalytic potential of ammonium halide salts was explored in the coupling reaction of a model aziridine with carbon dioxide, highlighting the superior activity of [NH2Et2]I. Then, working at room temperature, atmospheric CO2 pressure and in the absence of solvent, the [NH2Et2]I-catalyzed synthesis of a series of 5-aryl-2-oxazolidinones was accomplished in good to high yields and excellent selectivity, from 2-aryl-aziridines with N-methyl or N-ethyl groups. NMR studies and DFT calculations outlined the pivotal role of both the diethylammonium cation and the iodide anion. The proposed method represents a convenient choice for obtaining a limited number of valuable molecules for which more complex and more expensive catalytic systems have been reported even in recent years.