Sulfadiazine, a potent antibacterial agent belonging to the group of antibiotics called sulfonamides, has been reported to be present in surface and groundwater. This study investigated the degradation of sulfadiazine in a goethite (alpha-FeOOH) oxalate Fenton-like system under UV irradiation. The results showed that sulfadiazine could be effectively photodegraded by the goethite oxalate Fenton-like system as a result of the formation of the highly oxidizing hydroxyl radicals, center dot OH. Among the iron oxides tested (alpha-FeOOH, gamma-Fe2O3, gamma-FeOOH, and alpha-Fe2O3), alpha-FeOOH was found to he the most effective. Degradation of sulfadiazine depended significantly on the pH and initial concentration of oxalic acid in the system, with optimal values of 3.5 and 4.0 mM, respectively, under UV irradiation. Five intermediate products of sulfadiazine degradation were identified using high-performance liquid chromatography mass spectrometry (HPLC MS), gas chromatography mass spectrometry (GC MS), and ion chromatography (IC), and a possible sulfadiazine degradation pathway in such a system was proposed. Organic sulfur and organic nitrogen mineralization were also observed, and the results indicated that cleavage of the sulfonylurea bridge was easier than the other potential cleavage bonds under the goethite oxalate system. In addition, results from Biolog assays suggested that the ecological toxicity of the sulfadiazine solution was effectively reduced after degradation.