Oxidation of a dilute aqueous solution of a model azo dye pollutant (Orange II) was studied in batch and continuous well-mixed (CSTR) reactors. Both reactors operate at 200 - 250 degrees C, and total pressures up to 50 bar and at oxygen partial pressure from 10 to 30 bar. The model pollutant concentrations were in a range between 100 and 1,000 mg/L, which may be found in industrial wastewaters. The dye oxidation undergoes a parallel-consecutive reaction pathways, in which it first decomposes thermally and oxidatively to aromatic intermediates and via organic acids to the final product carbon dioxide. To develop a kinetic equation capable of predicting organic carbon reduction, all organic species present in solution were lumped by total organic carbon (TOC). The lumped oxidation rate in batch reactor exhibited second-order behavior, whereas in the CSTR it was found linearly proportional to its TOC concentration. The lump behavior in batch reactor was dominated by the refractory low molecular mass aliphatic acids formed during the oxidation.