Catalytic wet air oxidation (CWAO) has been investigated for the treatment of water that has been contaminated by 4-hydroxybenzoic acid (4HBA). Both batch measurements for kinetics determination and continuous trickle-bed operation have been performed on the same activated carbon (AC). After a fast initial deactivation, AC was proven to be stable and efficient at moderate temperature and oxygen pressure, such as that for phenol degradation. The kinetic study in the case of highly adsorbing material (such as AC) may require a complex approach to account for the variation of adsorbed reactants during batch oxidation. Adsorption isotherms at reaction temperature and with aged AC have been obtained, according to the Langmuir equation, and used in a 4HBA mass balance to derive more significant kinetic parameters. At high catalyst loading and relatively low 4HBA concentration, the variation of 4HBA during the batch oxidation may be even higher on the solid than in the aqueous phase. Some data in continuous trickle-bed reactor or upflow flooded-bed reactor indicate oxidation behavior similar to that observed in batch mode, despite very different liquid-solid ratios.