The presence of uronic acid residue in polysaccharide induces a difficult quantitative determination of monosaccharides. Reduction of uronic acid into corresponding hexose allows a complete monosaccharide liberation by acidic treatment. However, reduction methods previously described have not permitted a quantitative reaction. This paper describes a new quantitative reduction procedure for the uronic acid containing different polysaccharides. Uronic acid reduction is performed by methylesterification in dimethylsulfoxide/methanol with diazomethane followed by reduction in imidazol buffer with potassium borohydride. Moreover, the technique allows a quantitative reaction of low quantities of polymer without polymer degradation.