A novel oligosaccharide-labeling method involving the use of a detachable fluorescent tag has been developed based on the direct introduction of 4-methyl-7-thioumbelliferyl (MUS) group to the anomeric center of unprotected sugars. The resulting MUS-labeled sugars showed high sensitivity for fluorescence detection and could be used for quantification of oligosaccharides mixture. The unprotected sugars could be successfully recovered by a selective oxidative cleavage of the thioglycosidic bond under mild reaction conditions.