The in situ detection of electrical conductivity variation of amorphous tungsten trioxide (a-WO3) thin films during insertion and extraction of Li+ ions was carried out by microwave techniques coupled to electrochemical measurements. The techniques were applied to an electrochemical cell containing lithium electrolyte. The intensity of the transmitted microwave was measured, together with the potential of the cell during galvanostatic polarization. As expected, the electrical conductivity of a-WO3 increases during insertion and decreases during extraction of Li+ ions. The in situ study shows that if O2 evolution occurs at the a-WO3-electrolyte interface during oxidation after Li+ ions have been completely extracted, then a sudden decrease of conductivity is observed in the oxide layer, suggesting that oxidation of the sub-stoichiometric a-WO3 thin film into a more stoichiometric oxide may be undergone by an electrochemical process at room temperature.