The applicability of easily available ammonium salts to access aziridines via an ammonium ylide pathway was carefully investigated and compared with classical aza-Darzens approaches using α-bromo carbonyl nucleophiles. Whereas amide-stabilised ammonium ylides could be successfully reacted with aromatic aldimines to obtain the corresponding trans-aziridines in good yields and with high diastereoselectivities, α-bromo amides failed. In sharp contrast, acetophenone-based ylides did not give any aziridines while in this specific case α-bromo acetophenone derivatives gave the corresponding cis-aziridines in high yields and with excellent diastereoselectivities under optimised conditions.