An arrangement for introducing a liquid medium, e.g. urea, into exhaust gases from a combustion engine, which arrangement comprises: - a line intended to have exhaust gases flowing through it, - an injection chamber with an open forward end, - an injection means for injecting the liquid medium into the injection chamber, - a casing which demarcates the injection chamber in radial directions and is provided with throughflow apertures distributed round the circumference of the casing, - a mixing duct which forms part of the line and to which the injection chamber is connected via its open forward end, and - a gathering chamber which is situated upstream of the injection chamber, surrounds at least part of said casing, has an inlet to receive exhaust gases and is connected to the injection chamber via said apertures of the casing in order to allow the exhaust gases received in the gathering chamber to enter the injection chamber via these apertures, - a bypass duct which is situated in the line upstream of the mixing duct in order to lead exhaust gases into the mixing duct without passing through the gathering chamber and the injection chamber, and - that the inlet of the gathering chamber is adapted to diverting a portion of the exhaust gases flowing through the line, in order to cause these diverted exhaust gases to flow into the gathering chamber, thence into the injection chamber via said apertures and thereafter into the mixing duct via the open forward end of the injection chamber, while the bypass duct is adapted to leading another portion of the exhaust gases flowing through the line into the mixing duct in order to be mixed there with said diverted exhaust gases. characterised in that the combined cross-sectional area of the apertures of the casing is smaller than the cross-sectional area of the inlet of the gathering chamber so that the exhaust gases diverted to the gathering chamber are subject to a pressure increase in the gathering chamber. - a line intended to have exhaust gases flowing through it, - an injection chamber with an open forward end, - an injection means for injecting the liquid medium into the injection chamber, - a casing which demarcates the injection chamber in radial directions and is provided with throughflow apertures distributed round the circumference of the casing, - a mixing duct which forms part of the line and to which the injection chamber is connected via its open forward end, and - a gathering chamber which is situated upstream of the injection chamber, surrounds at least part of said casing, has an inlet to receive exhaust gases and is connected to the injection chamber via said apertures of the casing in order to allow the exhaust gases received in the gathering chamber to enter the injection chamber via these apertures, - a bypass duct which is situated in the line upstream of the mixing duct in order to lead exhaust gases into the mixing duct without passing through the gathering chamber and the injection chamber, and - that the inlet of the gathering chamber is adapted to diverting a portion of the exhaust gases flowing through the line, in order to cause these diverted exhaust gases to flow into the gathering chamber, thence into the injection chamber via said apertures and thereafter into the mixing duct via the open forward end of the injection chamber, while the bypass duct is adapted to leading another portion of the exhaust gases flowing through the line into the mixing duct in order to be mixed there with said diverted exhaust gases.