An exhaust line of a motor vehicle includes an upstream monolith and a downstream monolith for treating exhaust gases passing along the exhaust line. The upstream and downstream monoliths are positioned in series. An injection portion is positioned between an upstream face defined by the upstream monolith and a downstream face defined by the downstream monolith and includes a duct through which a stream of exhaust gas flows. The duct extends from the upstream face to the downstream face and has a central line of a set length between the faces. A reagent injector is mounted on the injection portion and is designed to inject reagent in gaseous form into the injection portion. At least one cup is positioned inside the duct in the path of the exhaust gas stream so that a mean path of the exhaust gas stream line is at least 20% longer than the set length.