Exhaust gas treatment assembly unit for an exhaust system of an internal combustion engine, comprising an exhaust gas/reactant mixing assembly unit and at least one SCR catalytic converter unit (48, 50) downstream of the exhaust gas/reactant mixing assembly unit and an assembly unit comprising an oxidation catalytic converter unit or/and a particle filter device upstream of the exhaust gas/reactant mixing assembly unit, said exhaust gas/reactant mixing assembly unit comprising an inlet area of an exhaust gas flow duct and a reactant release device for releasing reactant into exhaust gas flowing in the exhaust gas flow duct, wherein the exhaust gas flow duct comprises a mixing section with a first mixing section segment downstream of the reactant release device, wherein an exhaust gas/reactant mixture flows in the first mixing section segment essentially in a first main flow direction (H 1) in the direction from the reactant release device to a deflection area, and a second mixing section segment, wherein the second mixing section segment surrounds the first mixing section segment in an essentially ring-shaped manner, and the exhaust gas/reactant mixture flows in the second mixing section segment in a second main flow direction (H 2) essentially opposed to the first main flow direction (H 1) from the deflection area to an outlet area of the exhaust gas flow duct, wherein a tubular first ring wall is provided, elongated in the direction of a longitudinal central axis, extending in the first main flow direction (H 1) and defining the first mixing section segment radially outwardly in relation to the first main flow direction (H 1) and separating this mixing section segment from the second mixing section segment, and wherein a second ring wall is provided, extending in the second main flow direction (H 2) and defining the second mixing section segment radially outwardly in relation to the second main flow direction (H 2), wherein an inlet area (68, 70) of the at least one SCR catalytic converter unit (48, 50) and the outlet area of the exhaust gas flow duct of the exhaust gas/reactant mixing assembly unit are located in the same axial area of the mixing section in relation to the first main flow direction (H 1) and to the second main flow direction (H 2), wherein the outlet area of the exhaust gas flow duct opens into a releasing flow chamber guiding the exhaust gas/reactant mixture from the outlet area of the exhaust gas flow duct essentially radially outwardly to the inlet area (68, 70) of the at least one SCR catalytic converter unit (48, 50) in relation to the second main flow direction (H 2), wherein an outlet area of the assembly unit comprising the oxidation catalytic converter unit or/and the particle filter device is open towards a receiving flow chamber, and the inlet area of the exhaust gas flow duct is open towards the receiving flow chamber, characterized in that the inlet area and the outlet area of the exhaust gas flow duct of the exhaust gas/reactant mixing assembly unit are arranged essentially in the same axial end area of the exhaust gas/reactant mixing assembly unit, and that the assembly unit is received in an essentially cylindrical housing adjacent to the exhaust gas/reactant mixing assembly unit in relation to a central longitudinal axis and overlapping the latter.