The degradation of chlorothalonil was explored using UV, H₂O₂/UV, Oxalate/UV, and oxalate assisted H₂O₂irradiation. The optimum initial hydrogen peroxide and oxalate concentrations in the photolysis were about 100 mg L^-1 and 1 mM, respectively. The optimum pseudo-first-order reaction rate constants for UV (k_uv), H₂O₂/UV (k_H2O2), oxalate/UV (k_oxalate), and oxalate-assisted H₂O₂/UV (k_H2O2-oxalate) irradiation processes were 0.186, 0.357, 0.520, and 1.806 min^-1, respectively. A TOC analysis of the solution indicates that chlorothalonil is largely decomposed to carbon dioxide by the oxalate-assisted H₂O₂/UV irradiation process. In our experiments, the oxalate-assisted H₂O2/UV irradiation process was the most effective method for the degradation of chlorothalonil relative to the use of UV alone or the H₂O₂/UV irradiation processes.