The present invention concerns a method and a device for feeding at least one chemical substance into a main process stream. The method of the invention comprises the steps of providing a concentrated stream of said chemical substance, mixing said concentrated stream with a diluent stream to provided a diluted stream of said chemical substance, injecting said diluted stream of said chemical substance into said main process stream, wherein said mixing of said concentrated stream with said diluent stream is effected by injecting a jet of either one of said concentrated stream or said diluent stream into the respective other stream. The device of the invention comprises a first pipe segment (21) having a first inlet (22) for a first fluid and a first outlet (23) for a mixed fluid, a second pipe segment (24) having a second inlet (25) for a second fluid and a second outlet (26) connected to and leading into said first pipe segment (21) between said first inlet (23) and said first outlet (24), and first connection means (27) for connecting said first outlet (23) of said first pipe section (21) to a main pipe (14) of said main process stream, wherein said second pipe segment (24) comprises at least one nozzle (28) for generating a jet (29) of said second fluid arranged upstream of said second outlet (26).