Chemical disorder of battery active manganese dioxides (MD) evidenced as Mn3+, Mn vacancies and OH groups, is associated to well identified structural defects. Structural analysis of ordered-disordered MD with increasing defects content relates Mn3+to inter growth defects (Pr) and Mn vacancies to micro twinning (Tw). Potentiostatic reduction studies in 1N KOH provide I(V) and V(x) curves which show a correlation between the potential of the maximum intensity of the reduction current, the chemical and associated structural defects and the reduction kinetics of respectively EMD, CMD and ramsdellite. Reversibility is obtained for the two first forms in oxidation.