AbstractThe regioselectivity ratio RS = normal: abnormal opening of activated 2‐methylaziridines2by nucleophiles is found to range from 0.10 to unmeasurable large (only normal opening = substitution at CH2by strongly basic carbanions). RS is assumed to result from SN2 variants differing in the degree to which bond breaking is ahead of bond making including perhaps synchronous SN2. Bond breaking will be more ahead for the N‐CMe bond. High nucleophilic power pushes bond making toward a synchronous process resulting in great RS. The decrease in RS with acyl activation relative to sulfonyl activation is in accord with a flattening of the nitrogen pyramid (planarization effect). The planarization effect is retained in acidic medium by O‐protonation: Rs 0.10–0.14 for methanolysis as compared to RS 0.43 for N‐protonated sulfonylaziridine2h. AM1 calculations support the planarization hypothesis. – No indication for SET with trityl anio