A process for the preparation of an aziridine, especially ethyleneimine but also its derivatives in which one or both of its carbon atoms may be substituted once or twice by C.sub.1 -C.sub.8 -alkyl, C.sub.3 -C.sub.8 -cycloalkyl or aryl, and the nitrogen atom is substituted by hydrogen, C.sub.1 -C.sub.8 -alkyl, C.sub.3 -C.sub.8 -cycloalkyl, aryl benzyl, C.sub.1 -C.sub.8 -hydroxyalkyl or C.sub.1 -C.sub.8 - aminoalkyl wherein ethanolamine or its derivatives bearing the same carbon and nitrogen substituents is reacted in the gas phase for dehydration at from 200.degree. to 600.degree. C. and from 0.001 to 5 bar, especially 0.1 to 1 bar, over a heterogeneous catalyst shaped as presintered moldings and maintained as a steady-state or as a circulating fluidized bed catalyst. The preferred fluidizable presintered moldings are those prepared from a ceramic powder as a catalyst or catalyst carrier having a uniform pore size in the macroporous range.