This project determined the thermodynamic potentials for various reactions between reduced manganese (Mn²), manganese oxide (MnO₂(s)), chlorine dioxide (Cl0₂), and potassium permanganate (KMnO₄). Based on these findings, laboratory analyses were performed to determine if these reactions would occur under simulated water treatment plant conditions. In addition, a speciation procedure was developed to quantify the various species of manganese and chlorine dioxide present in a single sample. The reactions and the speciation procedure were evaluated at TOC concentrations ranging from < 1.0 mg/L to 5.0 mg/L and at pH 6.0 and 8.0. The speciation procedure yielded a reliable measure of Mn², insoluble manganese, and Mn⁺⁷; however, the Mn⁺⁷ evaluation could be disrupted by the presence of free chlorine. The determination of Cl0₂ and Cl0₂- concentrations was also possible; however, the C10₂- concentration was subject to error. The laboratory analyses revealed that Cl0₂ was unable to oxidize either Mn² or MnO₂(s) to Mn⁺⁷ under any of the thermodynamically favored conditions. Both KMn0₄ and Cl0₂ selectively oxidized reduced organic material before reducing the concentration of Mn². When C10₂ and KMnO₄ were added simultaneously, the ClO2 reacted preferentially with the reduced materials. Only after the Cl0₂ concentration was exhausted did the MnO₂⁻ begin to oxidize the reduced species. / Master of Science