A new mixed metal peroxo-polyacid based on Mo and V is formed by the direct reaction of a mixture of metallic Mo and V with hydrogen peroxide solution. A homogeneous amorphous thin film about 0.4 mu m is fabricated on an indium tin oxide (ITO) glass substrate by a spin-coating technique using this peroxo-polymolybdovanadate solution. After heat-treatment from 80 to 120 degrees C in air for 1 h, it shows reversible electrochromism in an organic LiClO4 electrolyte solution, and changes color from greenish yellow to grayish violet and violet, depending on the intercalation level of Li. Cyclic-voltammograms of the mixed Mo/V oxide film and those of the end members (MoO3 and V2O3) are quite different from one another, both in shape and peak-current potential, indicating that each film has distinctly different intercalation electrochemistry. The potential (E) vs. composition (x, Li content per mol Mo0.5V0.5O2.75) diagram of the present film recorded in the intercalation process agrees with that of the deintercalation process in the range 0 < x < 1.5. The E-x relationship is approximated by two straight lines with different slopes, with a kink at x = 0.5. This suggests that in this film there are two kinds of sites with different site energies. The changes in electrochromic properties with heat-treatment are also discussed in relation to the microstructure of the film.