Here are presented the results of electrochemical and optical measurements on tungsten trioxide, molybdenum doped (WO3:Mo) thin films, electrochemically deposited, assembled with Niafion-HR in order to realize an electrochromic (EC) device. The polymer that permits the ionic exchange has a residual viscosity, so that our device cannot be defined as a really solid state device. A good reason for the use of this polymer is its very high proton storage capability, so that, in our devices, it is possible to avoid the presence of a specific ion storage counterelectrode. In this work we compare also the electrochromic behavior of devices in dependence on the thermal treatment operated on the electrochemical films after deposition. Analysis has been carried on to reveal morphological characteristics of surface, thickness and stoichiometry of EC compound. Particular attention is paid to the potential waveform used to bias the devices, in order to optimize their performance and to avoid problems of stability and degradation.