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% H2O2and 9.7% concentrated H2SO4on 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 Eawere 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).