The electrochemical behavior of amorphous tungsten oxide (WO3) films is studied in various H3PO4/H2O binary mixtures including "pure" supercooled phosphoric acid (SCPA). It is shown that the kinetics of the bleaching-coloring transition is always fast with the same coloration efficiency in all solvents (57 cm(2) C-1 obtained from in situ transmission measurements). The water content has no significant influence on the voltammetric curves, and the redox reaction proceeds with the same diffusion coefficients of the proton in the range 10(-10) to 10(-11) cm(2) s(-1) calculated from chronocoulometric experiments. On the other hand, the stability of the WO3 films is tremendously improved in SCPA compared to the other more hydrated solvents. In this medium the electrodes are found to be remarkably stable upon continuous cycling at room temperature. The corrosion of the WO3 films in SCPA is also studied at higher temperatures. It is shown that the degradation of the WO3 films is considerably accelerated upon increasing the temperature.