In oxygen-free aquatic environments, such as ground water or hypolimnetic waters of eutrophic lakes, manganese predominantly as Mn(II) and natural organic compounds like humic materials occur. The effects of tannic acid and acetic acid as representatives of organic material on stabilizing the transformation of Mn(II) to Mn(IV) in a two-stage batch system have been examined. In the first stage, the catalytic effect of Mn(IV) linearly increased up to about 300 mg/l, keeping Mn(II) constant at 25 mg/l. In the second stage it has been studied how oxidation of Mn(II) and the catalytic effect of Mn(IV) are affected by tannic and acetic acids. Tannic acid completely inhibited the oxidation of manganese and acetic acid had no observable effect on the oxidation reaction.