Voltammetry and electrochemical quartz crystal microbalance (EQCM) studies of MnO2 films electrodeposited in acidic media at 85 degrees C show the existence of matter exchange during repeated electrochemical reduction and oxidation in 1 M KOH solution. Usually, these reactions are written without mentioning the exchange of matter. The following are observed : a reversible exchange involving water between the crystal lattice and the solution within each successive sweep potential cycle; an irreversible mass loss caused by low dissolution and slow diffusion of Mn(III) and Mn(II) solubilized species; an irreversible large mass gain during the first cycle due to additional hydration of the material.