Dyfonate is a dithiophosphonate pesticide used to control lepidopterous insects in corn, potatoes, and peanuts. It may be transferred to the surface water and underground water system and can be degraded by free chlorine and other oxidants, which are used by water treatment plants during the disinfection process, to produce oxidation byproducts. These byproducts may be more toxic than their parent compounds. Therefore, it is crucially important to identify these byproducts during the water treatment for full scale removal of them. In this article, a high pressure liquid chromatography/mass spectrometry (HPLC/MS) method was developed to investigate the oxidation byproducts of dyfonate by free chlorine (Cl2), hydrogen peroxide (H2O2), monochloroamine (MCA), chlorine dioxide (ClO2), Ozone (O3), and permanganate in an aqueous buffer (pH 7). Thin layer chromatography (TLC) and nuclear magnetic resonance (NMR) were also used for structure confirmation of the byproduct that was identified through HPLC/MS. One byproduct, dyfonate oxygen analog (phosphonothioic acid) was identified as the primary oxidation byproduct for free chlorine, ozone, MCA, and the H2O2 system, while no oxidation byproduct was detected in the ClO2 and permanganate system. The results of this study can serve as a valuable reference for water treatment plants.