Pentachlorophenol (PCP), a highly toxic phenolic compound, is a white or brown organic solid with needle-like crystals and a phenolic odor 1. The greatest use of PCP is as a wood preservative and biocide. PCP is one of the pollutants that have been placed on the United States’ Environment Protection Agency’s list of priority pollutants and subject to a stringent maximum contaminant level (MCL) of 0.001 mg L-1 set for drinking water. However, it is highly toxic, refractory, and hard to remove by conventional treatment processes 2,3. Ozone is a powerful oxidant and an effective purifier in water and wastewater treatment. Especially, ozonation treatment is widely applied to mineralize organic pollutants in water. However, when ozone is used alone, mineralization is not completely effective and also, some toxic and resistant byproducts (such as ketone, aldehyde, carboxylic acid, and bromate) are generated 4–6. To reduce these problems, advanced oxidation processes (AOPs) have been developed for the degradation of various classes of these organic compounds. Removing pollutants by AOP method is based on the production of free hydroxyl radicals with high oxidation potential which oxidize most of the organic compounds. Catalytic ozonation process (COP) is a new AOP method. In this process, for increased ozone decomposition a catalyst is applied, consequently forming highly reactive hydroxyl radicals 7. In COP, a variety of catalysts have been applied such as activated alumina 8, 9 magnesium oxides supported onto alumina 7, natural sand, and zeolite 10. Among these materials, aluminum oxide (-Al2O3, alumina), activated alumina, and metal oxides supported onto alumina have been used successfully in ozonation. Some catalysts such as -Al2O3, Fe2O3/Al2O3, CO3O4/Al2O3, and TiO2/Al2O3 have been used to accelerate the decomposition process of phenols, carboxylic acids, and phenolic compounds 11–14. Efficiency of a catalyst depends on the method of its preparation 13. According to some researches, it should be mentioned that the ozonation of PCP causes the production of some intermediate compounds, especially, tetrachloro-p-hydroquinone (TCHQ), tetrachloro-p-benzoquinone (TCBQ), and tetrachlorocatecol (TCCA) 15, 16. Since the reduction of byproducts during any treatment process is environmentally necessary, World Health Organization has established MCLs not only for parent compounds but also for their degradation byproducts in different steps and forms of water treatment 17. Knowledge on the formation of byproducts during ozonation would help optimize any post-ozonation unit by providing information on the composition of degradation products. In this work, PCP has been selected as a target compound. The main aims of this paper are to investigate the catalytic activity of -Al2O3 and confirm the byproduct formation during PCP ozonation.