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The surface of nanocrystalline cerium oxide (CeO 2 ) was treated with various chemical agents by a simple postmodification method at 25 °C and atmospheric pressure. Hydrogen peroxide, ammonium persulfate, deionized water, ascorbic acid, and ortho-phosphoric acid were used in order to study and evaluate their effect on surface materials, such as surface area, crystallite size, number of surface hydroxyl groups, particle morphology, and Ce 3+ /Ce 4+ ratio. Paraoxon-methyl (PO) decomposition and inorganic phosphate adsorption were used to evaluate the effect of surface treatment on catalytic and adsorption properties. CeO 2 surface was studied by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and acid-base titration. While the treatment procedure affected the number of surface hydroxyl groups and the amount of bulk surface oxygen vacancies, only negligible changes were observed in the Ce 3+ /Ce 4+ ratio. Interestingly, surface treatment affected the ability to decompose PO, but only a small effect on inorganic phosphate adsorption was observed, indicating the robustness of CeO 2 for the latter. A mechanism for possible interaction of the used chemicals with the CeO 2 surface was proposed.