Four series of cobalt-based catalysts, such as bare Co3O4 and CoO, CoOx-CeO2 mixed oxides, CoOx supported over alumina and alumina-baria and CoMgAl and CoNiAl hydrotalcites have been synthesized and investigated for the oxidative degradation of phenol in the presence of ozone. Characterizations were obtained by several techniques in order to investigate the nature of cobalt species and their morphological properties, depending on the system. Analyses by XRD, BET, TPR, UV-visible diffuse reflectance spectroscopy and TG/DT were performed. The CoNiAl hydrotalcite exhibits, after 4 It of reaction, the highest phenol ozonation activity followed by Co(3 wt%)/Al2O3-BaO and CoMgAl. The samples Co(1 wt%)/Al2O3-BaO and Co(1 and 3 wt%)/A1(2)O(3) show a comparable medium activity, while the oxidation properties of bare oxides Co3O4, CoO and CoOx-CeO2 are really low. Leaching of cobalt ions in the water solution was detected during the reaction, the amount varied depending on the nature of catalysts. A massive release was observed for the CoMgAl and CoNiAl hydrotalcites, while cobalt catalysts over alumina and alumina-baria look much more stable. The recycle of CoOx/Al2O3 and CoOx/Al2O3-BaO was studied by performing three consecutive cycles in the phenol oxidation. Because of the potential interest of the cobalt-supported catalysts in the ozonation process, the oxidative degradation of naphtol blue black was also investigated. On the basis of TPR and UV-visible results it appears that highly dispersed Co2+ ions especially present over Co(3 wt%)/Al2O3-BaO are the main active sites for phenol and naphtol blue black oxidative degradation by ozone. (c) 2007 Elsevier B.V. All rights reserved.