The wet air oxidation of phenol over a commercial active carbon catalyst has been studied in laboratory-scale and pilot-plant fixed-bed reactors at mild temperatures and oxygen partial pressures of 120-160 degrees C and 0.05-0.2 MPa, respectively. The performances of the fixed-bed reactors have been assessed and compared to each other for both up- and downflow operation mode. Depending on the flow mode and reactor scale, distinct phenol destruction rates have been observed in the experiments. A series of batch experiments are carried out to obtain phenol removal kinetics, which are subsequently implemented in the modeling of the pilot-plant fixed-bed reactor. A one-dimensional, nonisothermal piston dispersion model is developed to describe in detail the interplay of reaction kinetics, gas-liquid hydrodynamics, and heat and mass transfer in both flow directions. The model predicts reasonably well the experimental data, thus allowing for a thorough explanation of the observed pilot-plant reactor performance.