Methanesulfonyl chloride (MSC) forms a room temperature ionic liquid with AlCl3. The electrochemical properties of vanadium pentoxide (V2O5) films prepared by the sol–gel route were studied in this electrolyte. As a potential cathode, sodium is reversibly intercalated into the V2O5 film up to a stoichiometry of 1.6mole Na/mole V2O5 (−1V versus Al(III)/Al<1.5V) after the first discharge. The diffusion coefficient (D Na+ ) in the V2O5 film was determined to be between 5E−14 and 9E−12cm2/s using the potentiostatic intermittent-titration technique.