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Rechargeable batteries based on MnO2 cathodes, able to operate in mild aqueous electrolytes, have attracted remarkable attention due to their appealing features for the design of low-cost stationary energy storage devices. However, the charge/discharge mechanism of MnO2 in such media is still unclear and a matter of debate. Here, an in-depth quantitative spectroelectrochemical analysis of MnO2 thin-films provides a set of important new mechanistic insights. A major finding is that charge storage occurs through the reversible two electron faradaic conversion of MnO2 into water-soluble Mn2+ in the presence of a wide range of weak Brønsted acids, including the [Zn(H2O)6]2+ or [Mn(H2O)6]2+ complexes commonly present in aqueous Zn/MnO2 batteries. Furthermore, it is evidenced that buffered electrolytes loaded with Mn2+ are ideal to achieve highly reversible conversion of MnO2 with both high gravimetric capacity and remarkably stable charging/discharging potentials. In the most favorable case, a record gravimetric capacity of 450 mA·h·g-1 was obtained at a high rate of 1.6 A·g-1, with a coulombic efficiency close to 100% and a MnO2 utilization of 84%. Overall, the present results challenge the common view on MnO2 charge storage mechanism in mild aqueous electrolytes and underline the benefit of buffered electrolytes for high-performance rechargeable aqueous batteries.