A series of controlled current cycling experiments have been conducted to investigate the effect of barium hydroxide on the rechargeable performance of the alkaline manganese dioxide cathode. Analysis of the resulting discharge behavior revealed that the inclusion of Ba(OH) 2 suppresses the dissolution of Mn 3+ ions during the latter stages of discharge. This in turn gave rise to improved rechargeable performance, with electrodes containing Ba(OH) 2 exhibiting both improved capacity retention and higher cumulative capacities. Similar effects were observed during the cycling of manganese dioxide in the presence of soluble Ba(II) ions, and the implications in terms of its underlying role considered. In both cases barium hydroxide was also noted to inhibit the formation of δ-MnO 2 , and this was confirmed by X-ray diffraction and transmission electron microscopy.Electrochemical energy storage.-With the increased prevalence of portable electronic devices a higher demand is being placed on both the performance and efficiency of their electrochemical power sources. With global sales exceeding $10 billion per year, 1 batteries are the power source of choice due to their portability, reliability and excellent performance characteristics. Within this domain the alkaline Zn/MnO 2 system is prominent due to its low cost and strong performance in a wide variety of applications. While still widely regarded as a primary system, significant progress has been made in the development of rechargeable manganese dioxide (RAM) cells. 2-4 Despite the excellent performance of presently available cells, 5 there remains significant scope for improvement in terms of cathodic rechargeability under deep discharge conditions. Structure and discharge mechanism of γ-MnO 2 .-The preferred cathode material for use in both primary and secondary alkaline Zn/MnO 2 cells is that of electrolytic manganese dioxide (EMD). EMD has the γ-MnO 2 structure which is built up of edge-and cornersharing [MnO 6 ] octahedra in a manner consistent with a random microscopic intergrowth of pyrolusite (1×1) and ramsdellite (2×1) forms of MnO 2 . 6,7 Its structure has been considered in depth by Chabre and Pannetier, 8 who outlined a method for the calculation of the pyrolusite content (denoted P r ), and the level of microtwinning (denoted T w ) present in the (021) and (061) growth planes. Other key features associated with the γ-MnO 2 structure include cation vacancies, lower valent manganese cations (Mn 3+ ), and structural water present as protons associated with the oxide anions that compensate for the charge deficiency incurred by Mn 3+ ions and cation vacancies. 9-11