Peroxo-polymolybdotungstates are formed by the direct reaction of a mixture of metallic Mo and W (for example, 5:5 mixture) with hydrogen peroxide solution. A homogeneous and amorphous thin film (0.4 mum, typically) is fabricated on an ITO glass substrate by spin-coating using such a solution. After heat-treatment at 120-degrees-C, it exhibits electrochromism excellent in both reversibility and response time in an organic Li-electrolyte. The film colored at -1 V (versus Ag/AgCl) shows rather flat absorption spectrum. Its color is dark grayish blue and the DELTAOD between -1 and +0. 5 V reaches 0.7 at 670 nm (lambda(max)). Kinetic aspect of such electrochromic films will also be discussed.