The removal of 4-chlorophenol from aqueous phase continues to be an important environmental issue. In this work, the photochemical oxidation of 4-chlorophenol in aqueous solutions in a batch reactor using ultraviolet irradiation, hydrogen peroxide and nickel oxide was studied. The efficiency of the system was evaluated with respect to reaction time, pH, feed concentration of reactants, catalyst load, light intensity, and the reaction rate constant. The concentrations of 4-chlorophenol and chloride ions were determined by high performance liquid chromatography and ion chromatography, respectively. Pure nanosized nickel oxide was characterized by X-ray diffraction and scanning electron microscopy. The results showed that the optimum conditions (the complete 4-chlorophenol removal (100%) at 60 min) were obtained at a neutral pH, with 0.2 mol/L H₂O₂, and 0.05 g/L of nickel oxide. However, no pH effects were observed in the range of 4–10. Analytical profiles on 4-chlorophenol transformation were consistent with the best line fit of the first-order kinetics. Moreover, the degradation rate constant increased with both UV light intensity and decreasing initial concentration of 4-chlorophenol. Finally, the results of mineralization and chloride ions studies indicated that dechlorination was better accomplished but more time was required to completely mineralize 4-chlorophenol into water and carbon dioxide.