trans-3,4-Bis(trifluoromethanesulfonate)methylene-2,2,5,5-tetramethylpyrrolidin-1-yloxyl Radical is used in the synthesis of Trans-3,4-Bis[[(methylsulfonyl)thio]methyl]-2,2,5,5-tetramethylpyrrolidin-1-yloxyl Radical (B486250), which is a cross-linking sulfhydryl spin label.