The synthesis and properties of both epimers of 4-(N-hydroxyamino)-N-tosyl-Image -proline methyl ester and derivatives thereof are reported. In the presence of air, the compounds bearing a free N-hydroxyamino group oxidized in solution to give the corresponding aminoxyl free radicals, EPR spectra of which were Recorded. None of these compounds showed any interesting biological activity as such, but they constitute potential synthetic building blocks towards spin labeled glycopeptide or oligopeptide analogues.