The electrochromism of phosphomolybdic acid (PM) in solution and in a series of solid WO3-PM composites is investigated. PM in solution changes color from light yellow to dark blue upon reduction whereas the composites show a change from Light yellow to bluish brown under identical conditions. PM could not be deposited as a thin film by itself, and electrochemical codeposition was used to incorporate it into the WO3 matrix. The basic Electrochromatic properties of WO3 are changed upon the incorporation of PM even in small quantities. The composites have a more intense color, a different color spectrum, and a slower electrochromic response speed than that of WO3. The composites, like WO3, are also degraded by slow and continuous potential. cycling.