The wet air oxidation process (WAO) is an efficient method for treating wastewaters containing nitrogenous organic compounds, providing that a composite manganese/cerium oxide (7/3 molar ratio) is used as catalyst. This allows to obtain molecular nitrogen, absolutely harmless to the environment. During preliminary tests, it was established that the temperature (about 260°C) and the catalyst concentration are the most important factors to transform ammonia into molecular nitrogen (N-N2), which is the main product while only very limited amounts of nitrites (N- N02-) and nitrates (N- NO3-) appear. In addition, neither manganese nor cerium were leached off in the solution, under the experimental conditions. This composite oxide catalyst is adapted for the direct conversion of organic nitrogen (N-Org) to molecular nitrogen (N·N2), as well as for the conversion of organic carbon (TOC) to carbon dioxide. This was clearly established during the treatment of aliphatic nitrogenous compounds (-alanine, urea) and aromatic ones (aniline, aminophénol). Ammonia nitrogen (N-NH3) is always the main intermediate product and the rate of elimination of organic nitrogen is always rapid with regard to the second reaction step (transformation of N-NHJ to N-N2), which is the limiting one in this process. To account for the results obtained, it was possible to propose a general simple scheme with two main steps for the nitrogen transformation pathway and to determine the kinetic constants. This kinetic data is to be used in a next study about industrial reactors computation.