Treatment of primary and secondary amines with tert-butylmesitylenesulfonoxycarbamate and a base afforded tert-butoxyurea, which when treated with an acid ultimately yielded substituted N-hydroxyureas. It is proposed that this proceeded via the generation of t-butoxy isocyanate in situ. This method allows for the synthesis of both mono and disubstituted N- hydroxyureas.