Catalytic liquid-phase oxidation of aqueous solutions of substituted phenols (p-chlorophenol and p-nitrophenol) was studied in a differential, liquid-full operated fixed bed reactor. Experiments were performed in a wide concentration range of both reactants, i.e, dissolved model pollutant and oxygen, at reaction temperatures between 150 and 190 degrees C and total pressure of 30 bar. A proprietary catalyst containing supported copper, zinc and cobalt oxides was found to be effective for converting phenols via intermediates such as benzenedioles, quinones, and the C-4 compounds to carbon dioxide. A proposed intrinsic rate expression for disappearance of phenols is based on the Langmuir-Hinshelwood kinetic formulation, accounting for equilibrium model pollutant as well as dissociative oxygen adsorption processes on different types of active sites, and an initial hydroxyl hydrogen abstraction reaction to be the rate controlling step. The oxidation rates of phenols decrease as the Hammett constants of the substituents become more positive, reflecting trends in the basicity and nucleophilicity of substituted phenols. It is believed that heterogeneously catalysed aqueous-phase oxidation of phenols undergoes a combined redox and nonbranched-chain free-radical mechanism which involves initiation on the catalyst surface and, due to a high solid to liquid ratio in a fixed bed reactor, predominant heterogeneous propagation and termination. The involvement of a free-radical mechanism is indicated by the intermediates formed, and free-radical initiator and inhibitor effects on observed disappearance rates of phenols.