The hydrophobicity of polymer surfaces limits their applications in many areas such as for use as biomaterials and in membrane filtration. One solution to this problem is to modify the polymer surface by ozonation. Ozonation introduces peroxide groups on polymer surface, which can initiate graft polymerization of monomers with hydrophilic groups, and thus improves the hydrophilicity of the polymer surfaces. The concentration of peroxide groups formed can be used to indicate the effectiveness of ozonation process. In this study, the low cost polyethylene film was selected as a model polymer film to conduct the investigation. Ozonation treatment was carried out in both gaseous and aqueous phases, to study the contribution of hydroxyl radical in the generation of peroxide group. Results revealed that aqueous ozonation generated slightly less peroxide than gaseous ozonation. However, the addition of soluble catalyst, copper (II) sulfate, to the aqueous ozonation resulted in 18% more peroxide concentration than that yielded by gaseous ozonation. Further investigation indicated that 0.05 g/L copper (II) sulfate was the optimal catalyst dose, and the optimal pH was approximately 5.60. A 19% reduction in tensile strength of the film was observed after 120 minutes of catalytic ozonation. Upon addition of a radical scavenger, tert-butyl alcohol (TBA), a decrease of 12% in the peroxide concentration was observed for catalytic ozonation with 0.1 mol/L TBA. This decrease indicated that both ozone and hydroxyl radical contributed to the peroxide generation in catalytic ozonation. A reaction mechanism for aqueous ozonation of polyethylene was proposed in this study by combining the reaction mechanism for gaseous ozonation of polyethylene and the decomposition mechanism of ozone in water. The experimental data found in this study verified the exponential function obtained for peroxide concentration. This verification was obtained for various ozonation time and dose ranging from 15 – 120 minutes and 1.0 – 3.0 wt%, respectively.