The mechanism of the copper-catalyzed aziridination of alkenes using [N-(p-toluenesulfonyl)imino]-phenyliodinane (PhINTs) as the nitrene source has been elucidated by a combination of hybrid density functional theory calculations (B3LYP) and kinetic experiments. The calculations could assign a Cu(I)/Cu(III)-cycle to the reaction and demonstrate why a higher oxidation state of copper cannot catalyze the reaction. A mechanism whereby Cu(II)-catalyst precursors can enter the Cu(I)/Cu(III)-cycle is suggested. Three low-energy pathways were found for the formation of aziridines, where the two new N-C bonds are formed either in a nonradical concerted or consecutive fashion, by involvement of singlet or tripler biradicals. A close correspondence was found between the title reaction and the Jacobsen epoxidation reaction in terms of spin;crossings and the mechanism for formation of cis/trans isomerized products. The kinetic part of the study showed that the reaction is zero order in alkene and that the rate-determining step is the formation of a metallanitrene species.