The objectives were to evaluate permanganate and chlorine dioxide as preoxidants both when they were dosed individually and when they were dosed together, for their abilities to improve water treatment plant performance. The specific goals were to determine the effect of the preoxidant conditions on coagulant requirements, filter operation, and the removals of organic carbon, trihalomethane precursors, iron, and manganese. Also, the interaction of the preoxidants with each other when dosed together was investigated. The preoxidant conditions were evaluated on both plant- and laboratory-scale bases. The plant-scale study focused on the ability of the preoxidants to enhance those aspects of plant performance listed above. The laboratory-scale study reinforced the results of the plant-scale study and included investigations to the interactions between the two oxidants when they were dosed simultaneously. The plant-scale study yielded information regarding the extent to which these oxidants helped remove the raw water constituents listed previously. The laboratory-scale study expanded on these results and included information concerning the reasons why these oxidants performed in the manner that they did. During the period of lake stratification, when trihalomethane removal caused the greatest treatment concerns, chlorine dioxide provided the greatest removal efficiency (65 percent), but organic carbon removal suffered (45 percent), While the lake was mixing and reduced manganese posed the greatest treatment concerns, the simultaneous use of chlorine dioxide and permanganate resulted in the greatest removal efficiency (95 percent), however, objectionable tastes-and-odors were formed. In comparison, chlorine dioxide was more apt to oxidize organic compounds (including trihalomethane and taste-and-odor precursors), and permanganate was more apt to oxidize manganese(II). When the two oxidants were dosed simultaneously, chlorine dioxide had the capacity to maintain permanganate for extended periods, possibly beyond the hydraulic retention time of the plant. Methods were developed to measure 1) chlorite concentrations in the presence of oxidized forms of manganese and 2) the relative sizes and shapes of molecular—size distributions of naturally occurring organic matter. / Ph. D.