The invention relates to a device for disinfecting liquids by means of ultraviolet radiation in a continuous flow method, with a reactor chamber through which flow can take place and in which a number of UV radiator units is arranged, the reactor chamber being provided with a wall, which is open at two end sides, and locally surrounded by at least one pre-chamber. Advantageous flow conditions are achieved if the at least one pre-chamber is welded to the wall at a distance from the end side so that an annular intermediate space with an internal width (r) is formed between a side of the per-chamber and the wall, and an essentially axisymmetric annular gap (d) is formed between the end side and the pre-chamber.