The synthesis of a cellobiouronic thioglycoside donor12, protected with a selectively removable 3′-O-benzyl group is described. The donor12is suitable as a monomer building block in the construction of oligomer structures corresponding to the capsular polysaccharide ofStreptococcus pneumoniaetype 3. The carboxyl function was introduced through regioselective TEMPO-oxidation of a 4′,6′-diol cellobiose derivative. If the oxidation was performed on a 2,3,2′,3′,4′,6′-hexaol derivative, oxidation also of the secondary 2- and 3-hydroxyl groups was observed to give a tricarboxyl derivative as one of the major products. The thioglycoside was formed by acidic mercaptolysis of a 1,6-anhydro bridge. The donor12was transformed into a suitable starting monomer acceptor through glycosylation with a spacer alcohol and subsequent debenzylation.