We report a new material for self‐contained all‐solid‐state electrochromic devices with fast response. The material is phosphotungstic acid (PWA) which, at room temperature, is both an electrochromic material and an ionic conductor. Response times of 50 msec have been measured in cells of typeM/PWA/SnO2, whereMdenotes an electrode that provides ions of Ag, Cu, or Li. The cell exhibits memory.