This unit describes a collection of methods for chemical labeling of carbohydrates (free oligosaccharides or oligosaccharides conjugated to proteins, peptides, or lipids) by oxidation followed by reduction or by direct reduction. Oligosaccharides can be labeled with either radioisotopes or nonradioactive fluorescent molecules. These labelings allow one to follow the oligosaccharides during chromatography and in cells if labeled by fluorescent molecules. Selective oxidation with mild periodate followed by reduction with tritiated sodium borohydride results in selective radiolabeling of sialic acid residues on oligosaccharides or glycoproteins. Alternatively, treatment of samples with galactose oxidase results in oxidation of galactose or N‐acetylgalactosamine residues at nonreducing termini, rendering these residues suceptible to labeling with NaB[³H]₄. Oxidized glycoconjugates can also be labeled using the fluorescent probe lucifer yellow CH. Free oligosaccharides can be labeled by reduction with NaB[³H]₄. An additional protocol describes the release and simultaneous labeling of O‐glycan oligosaccharides by alkaline beta‐elimination in the presence of NaB[³H]₄.