Electrochemically active nanocrystalline tungsten oxide was synthesized in supercritical carbon dioxide from tungsten hexacarbonyl at 150 degrees C and 400bar in the presence of oxygen (partial pressure of 15bar). The supercritical fluid is a solvent for the precursor (i.e., this is a sc solvothermal synthesis route), whereas the admixed gaseous oxygen serves as an oxidizer, promoting thermal decomposition of the precursor. During the substrate-free synthesis, 200-500nm aggregates are formed. They consist of smaller grains having the size of about 100nm. Therefore, a certain structural hierarchy is detected. The electrochemical activity of the as-synthesized particulate material is pronouncedly increasing during both potential cycling and exposure in an aqueous aerated electrolyte. After such a hydration/oxidation process, the electrochemical response of the material shows rather fast and reversible recharging of the entire tungsten-containing phase. This is an indication of facilitated proton transport in bulk of the tungsten oxide phase synthesized in the supercritical carbon dioxide with subsequent hydration/oxidation. Quite differently, the material synthesized at the same temperature only in compressed oxygen (partial pressure of 15bar) without any presence of supercritical carbon dioxide is highly crystalline one. It does not demonstrate any significant electrochemical rechargeability; neither is the response improving with hydration/oxidation.