Reduction-dissolution kinetics and apparent activation energies (Ea) of reagent grade β-MnO2 (Fisher Scientific, Fair Lawn, NJ), β-MnO2 (Ward's Earth Science, Rochester, NY pyrolusite), and MnOOH (Ward's Earth Science, Rochester, NY manganite) were determined by continuous gasometric analysis employing an isothermal automanometric apparatus. This method was based on pseudo-first-order kinetics of oxygen gas (O2) evolution by reducing the Mn(IV) and/or Mn(III) to Mn(II). The manganese oxide reduction was carried out employing a solution mixture of 2.4% H2O2 and 9.7% concentrated H2SO4 on a volume basis. The pseudo-first-order constant (k) at 21°C was 0.66 min-1 for reagent grade β-MnO2, 1.68 min-1 for pyrolusite (β-MnO2), and 0.009 min-1 for manganite (MnOOH). The apparent activation Ea were determined to be 35.56 kJ/mol-1 for reagent grade β-MnO2, 29.95 kJ/mol-1 for pyrolusite (β-MnO2), and 44.75 kJ mol-1 for manganite (MnOOH). The most kinetically stable manganese oxide species of the three minerals tested was MnOOH (manganite).