The electrochemical reduction of four different forms of MnO2 (ramsdellite, beta-MnO2, gamma-MnO2 and epsilon-MnO2) was made in KOH 1N and 7N, under potentiostatic control. The structural disorder (De Wolff’s defauts and microtwinning) and stoichiometry defects are the origin of higher reduction potential observed for gamma- and epsilon-MnO2. The wide peak observed in the KOH 1N voltammogram for gamma- and epsilon-MnO2 is assigned to H+/e- insertion process, and not observed for ordered and stoichiometric forms ramsdellite and beta-MnO2. For KOH 7N, a step is defined in all cases and related to the dissolution of MnO2.