The invention discloses a method for detecting an activity of an endoglycosidase. The method includes providing the endoglycosidase with a substrate of the endoglycosidase and detecting cleavage of the substrate. The method further includes at least partly inhibiting the transglycosidase activity of the endoglycosidase. The transglycosidase activity may be inhibited by chemically modifying the substrate such that transglycosylation of the substrate by the endoglycosidase is at least partly inhibited while the endoglycosidase is still capable of cleaving the substrate. In one embodiment, the substrate comprises an oligosaccharide chain. Compounds and kits suitable for use in a method of the invention are also disclosed. Methods involving competitive inhibitors are also disclosed as are methods for the synthesis of glycosylated substrates involving the transglycosidase activity of an endoglycosidase.