Combined experimental and computational studies suggest that the iron-mediated aziridination of cis-1-phenylpropene proceeds along two mechanistic pathways that share a common imidoiron(IV) intermediate. One pathway involves a second species, proposed to be an azametallacyclobutane intermediate, which collapses to provide the syn-aziridine product. A second, parallel pathway is responsible for the formation of an anti-aziridine.