Aqueous phenol degradation and mineralization was carried out by using an atmospheric pressure plasma reactor operating under argon (Ar)/air plasma. The addition of CeO2, Ce0.90Ni0.10O2-δ and 10 wt% NiO/CeO2 catalysts improved the efficiency of phenol degradation. The modified ceria catalysts were prepared by wet impregnation and characterized by using X-ray diffraction (XRD), BET surface area measurements and Raman spectroscopy, to ensure the fluorite phase and oxygen-deficient sites. The effect of plasma gas, the concentration of the pollutant, applied voltage and the catalyst was studied. Typical results indicated that Ar plasma favors the degradation due to the formation of the highest amounts of oxidants like hydrogen peroxide (H2O2), which may be converted into hydroxyl radicals (OH) in the presence of a solid catalyst. Optical emission spectrum (OES) of the discharge confirms the formation of OH, whereas total organic carbon (TOC) analysis indicated the mineralization of the pollutant. © 2016 Elsevier Ltd. All rights reserved.