The major aspects of wet air oxidation and catalytic wet air oxidation have been reviewed in this work paying special attention to the reaction mechanisms, kinetics and the industrial process. In the experimental section a set of heterogeneous catalysts have been tested in the wet oxidation of non-wood black liquor. The oxidation runs were performed batchwise in a laboratory-scale mechanically stirred slurry reactor for 1 h at a temperature of 170°C and total pressure of 12 bar. Pure oxygen was used as oxidant agent. Results showed that Fe/AC, AC and CuO promote the formation of acids, whereas CeO2 was found to be effective for the complete oxidation of black liquor. It was also observed that highly concentrated streams cause catalyst deactivation due to active sites blockage.