Electrochemical, gravimetric and optical responses associated with Li+ insertion into WO3 were characterized in terms of complex transfer functions under an ac polarization regime. They provide additional information with respect to classical dc experiments. Various solvents and various lithium salts were investigated. Differences between propylene carbonate (PC) and ethanol are attributed to the appreciable contribution of perchlorate ions to the charge and mass exchanges in PC solutions. Electrogravimetric transfer functions show that this contribution has slow kinetics. The frequency response of the optical absorbance with respect to the charge and mass intercalated does not reveal any fast coloration process. The main limiting factor in the optical response to Li+ intercalation appears to be neither diffusion nor charge transfer but rather a rate-determining transition between two different kinds of Li+ incorporation in the WO3 lattice.