This experimental study was conducted to investigate the effect of treatment of landfill leachate by a coagulation-photooxidation process. The effects of different dosages of coagulant and different pH values on the coagulation processes were compared. The effect of different concentrations of sodium oxalate (Na2C2O4) on the treatment process was also studied after the coagulation was performed using FeCl3.6H(2)O. The experimental results show that in the pH range of 3-8, the lower the pH value, the higher the efficiency of the treatment. A 24% removal of COD (chemical oxygen demand, mg(O-2) l(-1)) can be attained by the addition of 1000 mg l(-1) FeCl3. A 31% removal of COD can be attained after 4 h of irradiation alone, and a 64% removal of COD can be attained after 4 h irradiation at pH 3 with the addition of 500 mg l(-1) FeCl3.6H(2)O. (C) 2002 Elsevier Science B.V. All rights reserved.