Catalyzed glycosylation of a galactosyl donor may allow many potentially useful and biologically important carbohydrates to be synthesized. However, the microscopic process behind it remains unclear. In this work, its core mechanism, a neighboring participation step, is computationally resolved. Seemingly this step involves only one reactant—the glycosyl donor; but surprisingly, it is catalyzed by another reactant—the alcohol-like glycosyl acceptor. That means that the reaction is actually bimolecular in nature.