Sulfoxides of Cys (S-p-methoxybenzyl) and Cys (S-benzyl) were prepared by oxidation with sodium perborate. Hydrogen fluoride and methanesulfonic acid converted the former sulfoxide to S-p-methoxyphenylcysteine or S-p-hydroxyphenylcysteine in the presence of anisole or phenol, respectively, while the latter sulfoxide resisted the actions of these deprotecting reagents. Thiophenol appears to be useful as a powerful reducing reagent for protected cysteine sulfoxides.