In the pyrolysis of threo-S-phenyl-S-l,l-diphenyl-2-methoxyethyl-N-ptosylsulfilimine the olefine isolated was not found to be 100% cis but contained 5.5% trans isomer 1) . This stereochemical result appears to suggest the possibility of thermal racemization of the sulfilimine at either S atom or C atom or both during the reaction. .f-er to examine the possibility of the thermal racemisation of sulfilimines, several optically active aryl methyl sulfilimines were prepared and subjected to heating in a few solvents. Actually these sulfilimines were found to undergo an extremely facile thermal racemization upon heating. Therefore, we wish to report briefly the results of this facile racemization reaction and discuss its plausible mechanism. Optically active N-p-tosyl aryl methyl and benzyl p-tolyl sulfilimines were prepared from the corresponding active sulfoxide and N-sulfinyl p-tosyl amide in pyridine at OยฐC according to the method by Day and Cram. 2) The optically active products were identified by comparing their i.r. and n.m.r. spectra with those of the racemic compounds. The optical activity was measured with semi-automatic polarimeter (Yanagimoto OR-10 ). The racemization reaction was carried out by dissolving the sulfilimine in benzene, chloroform or ethanol in the temperature range of 90-120ยฐC. The rate of racemisation was followed polarimetrically and found to obey the first 1377