The degradation of diphenylamine (DPA) in aqueous solution by persulfate is investigated. Effects of pH, persulfate concentration, ionic strength, temperature and catalytic ions Fe3+ and Ag+ on the degradation efficiency of DPA by persulfate are examined in batch experiments. The degradation of DPA by persulfate is found to follow the pseudo-first-order kinetic model. Increasing the reaction temperature or persulfate concentration may significantly accelerate the DPA degradation. Fe3+ and Ag+ ions can enhance the degradation of DPA, and Ag+ ion is more efficient than Fe3+ ion. However, the increase of either the pH value or ionic strength will decrease the rate of DPA degradation. N-Phenyl-4-quinoneimine, N-carboxyl-4-quinoneimine, 4-quinoneimine and oxalic acid are identified as the major intermediates of DPA degradation, and a primary pathway for the degradation of DPA is proposed. The degradation of DPA in surface water, groundwater and seawater is also tested by persulfate, and more than 90% of DPA can be degraded at room temperature in 45 min at an initial concentration of 20 mg L-1. (C) 2008 Elsevier B.V. All rights reserved.