Electrochemical impedance spectroscopy (EIS) has been used to examine the electrochemical and morphological (porosity) changes that the alkaline manganese dioxide cathode undergoes during reduction. Most significantly, within the EIS data a previously unreported intermediate-frequency (2-50 Hz) inductance loop was observed. This inductance loop, as well as the high (> 50 Hz) and low (< 2 Hz) frequency EIS data, was able to be interpreted in terms of changes in both the manganese dioxide and electrode porosity, as well as mechanical degradation of the manganese dioxide itself. The key finding was that for compositions more reduced than similar to MnO1.80, structural expansion has caused mechanical degradation of individual particles, as well as caused porosity changes within individual manganese dioxide particles and the electrode itself, essentially making them unavailable for discharge (proton insertion). A further implication of this is that the performance of the alkaline manganese dioxide cathode is then predominantly dependent on the geometric surface area and the bulk gamma-MnO2 structure. (c) 2006 The Electrochemical Society.