The use of oxidants in drinking water treatment is common for disinfection, oxidation of inorganic and organic contaminants, taste and odor control, and microflocculation. A wide variety of pesticides are often observed in drinking water raw and treated waters. It is well known that oxidation byproducts of synthetic organic compounds may have a greater, similar or lesser toxicity than the parent compound depending on the specific modifications occurring in the chemical structure. With a few notable exceptions, relatively little is currently known about which systems of oxidants and pesticides lead to significant parent oxidation and removal with concurrent formation of oxidation byproducts. In this AWWARF-funded study, the chemical reactivity of more than 70 pesticides with various oxidants (free chlorine, monochloramine, ozone, permanganate, chlorine dioxide and hydrogen peroxide), plus UV radiation and hydrolysis, is being examined under typical drinking water treatment conditions. In this presentation, selected systems of the most reactive and important (as determined by potential toxicity, occurrence, use, and other parameters) pesticides will be presented in detail including specific oxidation byproducts and pathways. The screening study is conducted using both GC/MS and LC/MS methods with degradate identification of byproducts conducted using ion trap LC/MS methods. Oxidation kinetic studies and parameter estimation of byproducts (e.g., partition coefficients, rate constants, etc.) are also determined in this study. In this paper, the primary oxidation byproducts and pathways for selected pesticides examined in detail to date will be presented including diazinon. Major byproducts were usually confirmed by use of standards, which also allowed detailed investigation of the subsequent oxidation of the byproducts themselves. Properties of the byproducts will be presented as determined using both experimental methods and quantitative structure activity relationships (QSAR). This work will guide the direction of future research with respect to occurrence, treatment, and toxicology of specific pesticide degradates in drinking water.