Motor vehicle exhaust line (910) of the type comprising: a hot pipe (912) for the exhaust gases; a cold pipe (914) for the exhaust gases; a mechanical decoupling element (920) connecting the downstream end of the hot pipe (912) to the upstream end of the cold pipe (914); a device for treating nitrogen oxides; and injection means intended to inject a reagent into, or to produce a reagent in the exhaust line (910) upstream of the nitrogen-oxide-treatment device. The invention is characterized in that the line comprises a mixer intended to mix the exhaust gases and the reagent injected or produced by the injection means, said mixer being positioned upstream of the nitrogen-oxide-treatment device. The nitrogen-oxide-treatment device is disposed in the cold pipe (914) downstream of the mechanical decoupling element (920), and the injection means and the mixer form an assembly connected directly to the mechanical decoupling element (920).