In this work a structural and porosity analysis of a series of chemically reduced electrolytic manganese dioxide ͑EMD͒ samples has been carried out. EMD, or ␥-MnO 2 , for use in aqueous alkaline cathodes possesses an orthorhombic unit cell that progressively increases in size as reduction continues. This unit cell expansion is nonuniform, with dimensional changes being different in all three dimensions ͑b 0 Ͼ a 0 Ͼ c 0 ͒. The unit cell expansion is the direct result of the reduction process, with the discharge products ͑Mn 3+ and OH − ͒ being larger than the corresponding ions in the host lattice ͑Mn 4+ and O 2− ͒. Gas adsorption isotherms on the same series of reduced EMD samples demonstrated that the Brunauer-Emmett-Teller surface area decreased considerably over the entire discharge compositional range. The calculated pore size distribution showed that microporosity was eliminated with reduction, but the mesopore volume increased. The origin of these changes in porosity is discussed in terms of the structural expansion, as is their effect on electrochemical performance.Background.-Electrolytic manganese dioxide ͑EMD͒ is a porous material 1-3 with Brunauer-Emmett-Teller ͑BET͒ surface areas ranging from 10 to 100 m 2 /g, although commercially produced materials are typically in the range 25-35 m 2 /g. These large BET surface areas are the result of the EMD containing a large amount of internal porosity, covering a broad range of pore sizes from micro-, through meso-, to macropores. 1 Changes to the conditions used to manufacture EMD alter this pore size distribution, and given the wide range of BET surface areas that an EMD can possess, the variation in pore size distribution can be substantial. The implications of this on the electrochemical behavior of the EMD are also quite significant, given that not all of this porosity may be accessible to the electrolyte, meaning that the electrochemically active surface area is almost always going to be different compared to the BET surface area. 2 Manganese dioxide has been used as the cathode-active material in primary batteries for many decades. EMD is the active material of choice, and within an alkaline cathode it is mixed intimately with an electronic conductor such as graphite and an ionic conductor such as a concentrated aqueous KOH solution. 2-4 This electrode configuration generates a high surface-area-to-volume ratio, which results in a large electrolyte-electrode interface, thus allowing its efficient use as a cathode. 2 As performance of the alkaline EMD cathode is a limiting factor for alkaline batteries at high drain rates, 2 gaining a better understanding of how electrode features such as porosity effect mass-transport phenomena occurring near this interface is desired. Despite many years of use, and research into its behavior, there are still many features of manganese dioxide behavior that need clarification. One of these is the role of manganese dioxide porosity, which is the subject of this work.