We present here the design and the characterization of a three-electrode electrochromic device (ECD) based on the WO3/IrO2 system. An Ag/AgCl pseudoreference was designed to be incorporated in the thin polymeric layer of electrolyte separating the electrochromic WO3 electrode from the IrO2 counter-electrode. This reference electrode enabled an in situ characterization of the WO3 and IrO2 materials, thereby helping in understanding the electrochemical mechanisms arising at each electrode during cycling. The reliability of such three-electrode measurements was also demonstrated by comparing the results obtained in liquid electrolyte with those obtained in the thin film polymer-based ECD.