Novel cyclic oligomers of the formula ##STR1## ARE PREPARED BY CONTACTING (A) A STRONG PROTIC ACID WITH (B) AN N-substituted non-activated aziridine of the formula ##STR2## wherein R.sub.1 and R.sub.2 are independently hydrogen or lower alkyl and R.sub.3 is hydrogen, hydroxyl, cyano or an organic radical which is inert in the process. The process is typically conducted in an aqueous alkanol medium. E.g. the cyclic tetramer of N-phenethylaziridine was obtained as the predominant product by heating a solution of N-phenethylaziridine in aqueous ethanol at reflux temperatures in the presence of p-toluenesulfonic acid.