Chemical oxidation of produced water separated from gas stream of a gas refinery in Iran, has been attempted by use of hydrogen peroxide, ozone and calcium hypochlorite. All the experiments were carried out at 30 °C. The aim of the current work was improving the quality of produced water via degradation of soluble organic materials in order to be re-used as fire-fighting water supply source in the refinery. Produced water separated from natural gas at different stages of gas production process was collected and de-oiled in an API separator. The water effluent of API separator contained chemical oxygen demand (COD) and total dissolved solids (TDS) of 270 and 3450 mgL ^–1, respectively. The stoichiometry amount of hydrogen peroxide for complete COD removal of produced water (600 mgL ^–1) of hydrogen peroxide at pH of 7.2 decreased its COD to the level of 228 mgL^–1 which corresponds to 15% degradation of organic materials. Ozonation of produced water for 1 hour at pH of 7.2 ended to COD value of 203 mgL^–1 (i.e. 12% COD removal). Increasing pH of the water to 10, improved treated water quality (to COD of 192 mgL ^–1), which indicated higher ozonation efficiency of the produced water at higher pH values. Maximum COD removal was achieved for calcium hypochlorite oxidant. Employing concentrations of 300, 500, 1000 and 7100 mgL^–1 of calcium hypochlorite showed COD removal in the range of 36-70%. The final COD values were obtained as 80-173 mgL ^–1. Kinetic studies of calcium hypochlorite oxidation of produced water revealed that the reaction was completed within 30 minutes at concentrations higher than 500 mgL^–1. The results indicated that calcium hypochlorite was the most efficient oxidation agent among the three oxidants employed in this study. The drawback of using calcium hypochlorite is the high concentrations of residual active chlorine in the treated water which should be eliminated before using as fire-fighting water source.