A method of treating pollutants contained in exhaust gases, notably of an internal-combustion engine, with a system comprising an exhaust line with selective catalytic reduction catalysis means traversed by said gases and injection means for injecting a reducing agent into said line so as to treat said pollutants as they flow through said catalysis means, characterized in that it consists in:
- heating an organonitrogen compound so as to decompose it into a mixture of at least an ammonia-containing reducing agent in the gas phase, another reducing agent in the gas phase and steam,
- compressing, then cooling this mixture so as to condense the steam to a liquid water phase and to obtain a gas phase of one of the two reducing agents and a liquid phase of the other reducing agent,
- injecting one of the reducing agents into the exhaust line in combination with at least said catalysis means in order to treat the pollutants of these gases.