The reaction mechanism of α‐MnO2 having 2×2 tunnel structure with zinc ions in a zinc rechargeable battery, employing an aqueous zinc sulfate electrolyte, was investigated by in situ monitoring structural changes and water chemistry alterations during the reaction. Contrary to the conventional belief that zinc ions intercalate into the tunnels of α‐MnO2, we reveal that they actually precipitate in the form of layered zinc hydroxide sulfate (Zn4(OH)6(SO4)⋅5 H2O) on the α‐MnO2 surface. This precipitation occurs because unstable trivalent manganese disproportionates and is dissolved in the electrolyte during the discharge process, resulting in a gradual increase in the pH value of the electrolyte. This causes zinc hydroxide sulfate to crystallize from the electrolyte on the electrode surface. During the charge process, the pH value of the electrolyte decreases due to recombination of manganese on the cathode, leading to dissolution of zinc hydroxide sulfate back into the electrolyte. An analogous phenomenon is also observed in todorokite, a manganese dioxide polymorph with 3×3 tunnel structure that is an indication for the critical role of pH changes of the electrolyte in the reaction mechanism of this battery system. The pH matters: Investigation of the reaction mechanism of tunneled manganese dioxide with zinc ions reveals that contrary to the conventional belief that zinc ions intercalate into the tunnels, a series of conversion reactions involving active manganese dissolution and concomitant electrolyte pH change lead to the reversible formation of layered zinc hydroxide sulfate.