The present invention relates to a method of treating exhaust gas pollutants, notably of an internal-combustion engine, with a system comprising an exhaust line (L) with selective catalytic reduction catalysis means (62) and injection means (82) for injecting a reducing agent into the line so as to treat the pollutants, 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 said catalysis means (62) in order to treat the pollutants of these gases.