Abstract: The invention is a process for the preparation of N-substituted imidazolidinones and N-substituted 2-thionimidazolidinones which comprises contacting an oxazolidinone with a compound containing a nitrogen directly bonded to a carbonyl or a thiocarbonyl group in the presence of a Lewis acid catalyst or the hydrate of a Lewis acid catalyst under conditions such that an N-substituted imidazolidinone or N-substituted 2-thionimidazolidinone is prepared. The compound containing a nitrogen directly bonded to a carbonyl or a thiocarbonyl group is an isocyanate or isothiocyanate or a compound wherein the nitrogen is reactive and the carbonyl or thiocarbonyl group is further bonded to a substituent by a bond which is clevable under the reaction conditions. The Lewis acid catalyst corresponds to the formulaMX.sub.nwhereinM is a group IB-VIIIB, IIIA or IVA element with the proviso that M is not C or Si;X is a halogen; andn is 2, 3 or 4.