The catalytic ozonation of RR198 Solution in the presence of MgO nanocrystal catalysts was investigated in a laboratory scale batch reactor. The effects of solution pH (2-12), reaction time, MgO dosage (1-6 g/L), and initial dye concentration (100-500 mg/L) on color and COD removal were evaluated, and the findings were compared to those of ozonation without a catalyst. The results indicate that adding MgO nanocrystals into the ozonation reactor greatly accelerated the rate of RR198 degradation, thereby reducing the reaction time and improving the reduction of color and COD compared to conventional ozonation. The optimum pH and catalyst dosage values were determined to be 8 and 5 g/L, respectively. The complete removal of color was observed in COP at this optimum condition for an RR 198 concentration of 200 mg/L at a reaction time as short as 9 min, while the time required to attain the same performance at single ozonation was 30 min. Furthermore, the COP could markedly increase the ratio of BOD5 to COD from below 0.1 in raw solution to 0.63 to 0.38 for dye solutions of 100 and 500 mg/L, respectively; thus, RR 198 was converted to biodegradable compounds. Therefore, the COP on MgO nanocrystals is considered as an effective and feasible process for pre-treating the azo dye-laden solutions, making possible a post-treatment of the effluent in a biological system. (C) 2009 Elsevier B.V. All rights reserved.