1-Oxyl-2,2,5,5-tetramethyl-∆3-(methanesulfonyloxymethyl)pyrroline is an intermediate in the synthesis of 3-Iodomethyl-(1-oxy-2,2,5,5-tetramethylpyrroline)-15N the isotope labelled analog of 3-Iodomethyl-(1-oxy-2,2,5,5-tetramethylpyrroline) (I709500); a thio-reactive spin label.