Phosphotungstic acid was used at different hydrogen ion concentrations and combined with glycogen and lysozyme, which were used as model systems for carbohydrates and proteins, respectively. Furthermore, in order to correlate the chemical analysis with the selectivity of the "staining" reaction, ultrastructural studies were carried out using mouse skin, aorta, esophagus and colon. When the heteropolyacid was used at a pH around 1.0, and not above 1.8, there was no interaction with glycogen whereas an extremely high lysozyme binding ensued. At these levels of hydrogen ion concentration the selectivity of the staining was quite apparent. The rise in pH of the phosphotungstic acid solutions determined a loss of specificity in the staining reaction accompanied by a consistent glycogen uptake. This anomalous reaction was interpreted as the result of a decomposition of the metal into two altogether different chemical compounds: phospho-11-tungstic acid and free tungstates. The latter, like the molybdates, are known to be excellent complexing agents with carbohydrates.