X‐ray diffractional (XRD) studies were carried out for the electrochemical reduction of heat‐treated electrolytic (250° and 400°C for 7 days). A series of XRD examinations indicated that the reaction proceeded without the destruction of the core structure of electrolytic manganese dioxide (EMD). The structural changes during the electrochemical reductions of both EMDs were described, assuming a tetragonal sublattice. Both heat‐treated electrolytic (HEMDs) behaved similarly in the tetragonal sublattice parameter vs. the reduction degree plots. During the first half of the reduction, HEMD phase having a tetragonal sublattice ( , ) was converted into a new phase having an expanded tetragonal sublattice ( , ), i.e., in a two‐phase reaction. In the 30–90% reduction, the a‐axis increased continuously as a function of reduction degree, i.e., in a homogeneous phase reaction. The possible crystal structure of the deep discharge product is discussed, assuming a tetragonal unit cell ( , ) having the structure by analogy with , and an orthorhombic unit cell, i.e., , , . From these, assuming an orthorhombic unit cell having , , and , all diffraction lines from the reduction products of HEMD were indexed.