Phosphotungstic acid (PTA) used at different concentrations in water was combined with various compounds (amino acids, proteins, acid glycosaminoglycans, glycoproteins and simple sugars). The results obtained indicated that PTA interacts with positively charged groups, that the sugar-hydroxyls do not take part in the interaction and that the type of binding involves electrostatic forces. To substantiate these findings further, heparin and heparin oligosaccharides were N-desulfated, combined with PTA and the precipitates were analyzed. It was found that the interaction had taken place only between the metal and the desulfated hexosamine amino groups and that when the N-desulfated product was acetylated no PTA binding occurred. When PTA was used at higher concentrations and in extremely acidic conditions, a precipitation of proteins, amino acids, acid glycosaminoglycans and neutral sugars ensued. This complex formation was interpreted as due to ionic interaction between the polyacid and the protonated hydroxyl groups.