t-Butyl bromide-activated dimethyl sulphoxide reacted with carboxylicacids and differently N-protected aminoacids in the presence of a base (NaHCO3 or Et3N). High to quantitative yields of methylthiomethyl (MTM) esters were obtained in all cases without interference by other functional groups or racemization. The mechanism of the reaction is discussed on the basis of the products formed, of spectral