Optical and electrical properties of transmissive solid state electrochromic devices based on a sandwich structure of conductive K-glass(R)(1) /WO3/polymeric electrolyte/conductive K-glass(R), have been investigated in correlation with the type of salt added to the polymer in order to increase its ionic conductivity. Lithium, sodium and ammonium triflate salts have been used with poly (ethylene glycol) ethylethermethacrylate (pPEGMA) to obtain transparent and conductive semi-solid solutions. The physical properties of the three different types of electrolytes and the operational performances exhibited by devices in terms of colour contrast, response time and stability are discussed. The colouring and bleaching mechanism and the electrochromic behaviour appear to be very similar in the three cases, but a rapid optical degradation is present for devices made up with the ammonium triflate salt when they are operated under cyclic biasing condition. (C) 2000 Elsevier Science BN. All rights reserved.