A stable sulfonylimino-lambda(3)-bromane, CF3SO2N--Br+C6H4-p -CF3, has been syntesized and structurally characterized for the first time. X-ray crystallographic analyses indicated a centrosymmetric dimer structure with little double-bond character for the "ylidic" Br-N bond. THe iminobromane serves as an efficient imido group donor and directly u ndergoes aziridination of olefins stereospecifically of alkenes under transition-metal-free conditions. The observed rate constants for aziridination of cis -cyclooctene are proportional to concentration of the olefin. The results suggest the involvement of a bimolicular transition state, in which an olefin attacks the sigma(*) N-Br orbital of iminobromane in the nitrenoid transfer process.