A method and apparatus for mixing at least one gaseous fluid stream with a large gas stream flowing in a gas duct, especially for introducing a reducing agent into a flue gas containing nitrogen oxides. The gas stream is directed against at least one disk-shaped mixer element on an inlet side that is inclined at an angle counter to the direction of flow of the gas stream, wherein eddy-type whirls form at the mixer element. The gaseous fluid stream is guided essentially to the center of an impact surface associated with the discharge side of the mixer element and is admixed with a gas stream downstream of the mixer element and is guided essentially perpendicularly onto the center of the impact surface on the discharge side such that the gaseous fluid is distributed over the entire discharge side from the center thereof and is incorporated across the entire whirl system formed at the peripheral edge of the mixer element.