Production of aziridines comprising reacting an ethylenically unsaturated compound, such as styrene or methyl cinnamate, with a nitrene donor, such as (N-(p-tolylsulphonyl) imino) phenyliodinane in the presence of an acidic zeolitic material having a pore size large enough for the reactants to enter, and the aziridination product to leave, the zeolite supercage, said zeolite having been impregnated, or preferably exchanged, with ions of at least one metal selected from Groups VIA, VIIA, VIII, IB and IIB of the 4th to the 6th periods of the Periodic Table. The metal is preferably copper and the zeolite is preferably zeolite Y. Asymmetric aziridines may be made by treating the catalyst with a chiral modifier such as a 4,4'-disubstituted bis(oxazoline) before contact with the nitrene donor.