Process for treating nitrogen oxides in exhaust of an internal combustion engine

The invention relates to a method for treating nitrogen oxides in an engine equipped with a nitrogen oxide trap followed by a selective reduction catalyst for nitrogen oxides. The trap is purged on a mass threshold of stored nitrogen oxides. According to the invention, the type of mass threshold permitting the purge is continuously adjusted as a function of the temperature range in which the catalyst temperature is located. When the catalyst is fully hot, the mass threshold is set to a value which is much lower than the maximum nitrogen oxide storage capacity of the trap. Thus, in the case of a cold restart, when the catalyst is totally ineffective, there is always a reserve of storage capacity of the trap which allows treatment of the nitrogen oxides by the single trap, during the heating phase catalyst.