It was found that in the case of N‐(diisopropylphosphoryl) glutathion (reduced form), 2, N→ S phosphoryl migration took place, but not for N,N‐bis(diisopropylphosphoryl) glutathion (oxidized form) or N‐diisopropylphosphoryl cysteine. These results were deduced by 31P‐NMR tracing experiments. It was shown that phosphoryl migration was catalyzed by an intramolecular carboxyl group, and a mechanism involving a mixed carboxyl‐phosphoric anhydride was proposed. A competitive reaction between the amino and thiol group toward diisopropyl phosphite indicated that the phospho‐thiol derived from N‐(diisopropylphosphoryl) glutathion (reduced form), 2, did not result from direct phosphorylation of the thiol group. N, S‐Bis(diisopropylphosphoryl) glutathion provides an authentic sample to confirm the migrated phosphoryl thiol product.