Concurrent utilization of voltammetry and electrochemical quartz crystal microbalance methods for studying chemically deposited MnO2 films in alkaline media shows the existence of matter exchange during repeated electrochemical reduction/oxidation reactions. As already observed with electrodeposited films, the mass variations during reduction and oxidation correspond to exchanges higher than 150 g per faraday. These large exchanged masses can be explained assuming that the film has an open microporous structure in which the micropores are emptied when the film is reduced to Mn(OH)(2) and filled by water molecules, K+ ions, and dissolved Mn polyions as the film is reoxidized to MnO2. Finally, the MnO2 film acts as an electrochemical sponge. The sponging-up phenomenon is due to the combined effect of the dissolution/deposit process and the conductivity variation with the applied potential.