Transition-metal-exchanged zeolite Y (Cr, Mn, Fe, Co, Ni, Cu, Zn) are compared as catalysts for the aziridination of styrene using (N-(p-tolylsulfonyl)imino)phenyliodinane (PhI=NTs) as the nitrene precursor. The Cu-exchanged zeolite shows high levels of aziridine formation, but significant yields of aziridine are also obtained with the Cr-, Mn-, Fe- and Co-exchanged zeolite Y. In contrast, these cations produce much lower yields of aziridine in the corresponding homogeneously catalysed reactions. Addition of a chiral bis-oxazoline ligand leads to a significant decrease in the yield of aziridine for all the ion-exchanged zeolites, with the exception of the Cu-exchanged zeolite Y. Further experiments with (N-(p-nitrophenylsulfonyl)imino)phenyliodinane (PhI=NNs) as nitrene donor indicate that, in addition to Cu-exchanged zeolite Y, significant yields of the aziridine can be formed for Zn-, Mn, Fe- and Co-exchanged zeolite Y. The major by-product was benzaldehyde, probably formed by oxidation of styrene. These data confirm that Cu-exchanged zeolite Y is the best aziridination catalyst but it is shown that other metal-exchanged zeolites are also catalytically active.