Mutant glycosidase enzymes are formed in which the normal nucleophilic amino acid within the active site has been changed to a nonnucleophilic amino acid. These enzymes cannot hydrolyze disaccharide products, but which can still form them. Using this enzyme, oligosaccharides are synthesized by preparing a mixture of an &agr;glycosyl fluoride and a glycoside acceptor molecule; enzymatically coupling the &agr.glycosyl fluoride to the glycoside acceptor molecule to form a glycosyl glycoside product using the mutant glycosidase enzyme; and recovering the glycosyl glycoside product. Particular enzymes include a mutant form of Agrobacterium &bgr.Glucosidase in which the normal glutamic acid residue at position 358 is replaced with an alanine residue.