Abstract: A fluid mixing device, especially for treating super heated steam comprises a conduit (10) for a first fluid with a dispersing member (14) for a second fluid, such as water, arranged so that an annular space (16) is defined between the dispersing member (14) and the internal surface of the conduit (10). The dispersing member (14) comprises first and second parts (24, 26) mounted for relative movement. An outlet (62) for the second fluid is defined between the first and second parts (24, 26). The site of the outlet (62) is defined by relative movement between the parts (24, 26). The relative movement is determined by the rate of flow of the first fluid through the conduit (10).