Various polymer electrolytes based on boronate esters and different lithium and sodium salts have been evaluated in electrochromic (EC) devices based on WO3 films. The results showed that the ionic conductivity of the electrolytes was not the most important parameter for the colouration performance of these devices. The use of solid electrolytes containing LiClO4 resulted in a higher colouration performance than the corresponding liquid electrolytes, even though the latter exhibited a significantly higher conductivity. The results also showed that the transfer process at the interface between the WO3 layer and the electrolyte played a major role for the colouration process. The presence of Lewis acidic boronate ester groups in the electrolytes significantly improved the performance of the EC devices. The incorporation of boron in the composition of the electrolytes allowed the use of solid electrolytes instead of liquid ones, thus avoiding leakage problems. Furthermore, the highest colouration performance was found in EC devices operating with inexpensive salts. Because of their poor stability, the electrolytes based on LiCl and LiBr were not suitable, while those incorporating LiClO4 salt exhibited excellent overall characteristics. (c) 2006 Elsevier B.V. All rights reserved.