Substitution of the participating group of glycosyl donors by a halogen atom is shown to specifically induce degradation of transient orthoesters formed during glycosylation reactions, depending on the nature of the acceptor, and to affect the protonation profile of those intermediates. Following these findings, bromo- and chloroacetates, which are major protecting groups in glycochemistry because they are orthogonal to other esters, should be considered with care as participating groups in the future.