(1-Oxyl-2,2,5,5-tetraethyl-∆3-pyrroline-3-methyl) Methanethiosulfonate-d20 is labelled (1-Oxyl-2,2,5,5-tetraethyl-∆3-pyrroline-3-methyl) Methanethiosulfonate (O875005), an analogue of (1-Oxyl-2,2,5,5-tetramethyl-∆3-pyrroline-3-methyl) Methanethiosulfonate (O875000) which is a specific conformational probe of thiol site structure by virtue of its minimal rotational freedom and distance from the covalent disulfide linkage to the macromolecule under study.