Ac potential-modulated optical transmittance (color impedance) and electrochemical impedance measurements were employed in an in situ investigation of the electrochromic behavior of evaporated amorphous tungsten trioxide (a-WO3) films in non-aqueous (LiClO4 + propylene carbonate), aqueous acid (H2SO4) and mixed non-aqueous-aqueous acid (PC + HClO4) electrolytes. The optical and electrochemical responses were essentially the same with the 0.8 mu m thick films that were examined, indicating that all of the injected charge was involved in the faradaic process. The behavior of the WO3 films in LiClO4 + PC and H2SO4 electrolytes was relatively simple compared to that of the PC + HClO4 electrolyte, in which the charge transfer resistances and diffusion coefficients were found to vary considerably with potential, probably due to a shift from proton insertion at more positive potentials to lithium ion insertion at more negative potentials. In all of the electrolytes, evidence was seen for two types of diffusion processes, a slower one, most likely due to diffusion within small clusters of WO6 octahedra, and a faster one, due to diffusion in nanometer-scale spaces between these clusters.