A new method for the activation of thioglycosides was developed by use of benzeneselenenyltriflate (PhSeOTf), which, upon reaction with either a primary or secondary sugar HO-group, afforded O-glycosides under extremely mild reaction conditions. The reaction was applicable to various l-thiohexosides 2-6 as well as to a 2-thioglycoside 20 derived from N-acetylneuraminic acid (NeuAc).