A method and an apparatus are described for use in the irradiation of fluids. The apparatus has an elongate conduit (312) having a central axis (334) and two or more elongate ultraviolet radiation sources (114) extending along the interior of the conduit for irradiation of the fluid within the conduit. An array of static mixer elements (300) is located within the conduit, and the two or more elongate ultraviolet radiation sources (114) are arranged to extend through apertures in deflection surfaces of the static mixer elements making up the array (300). The apparatus and method allows for reliable and uniform ultraviolet irradiation of fluids of low UV transmissivity, such as turbid fluids, particularly for ultraviolet disinfection of such fluids. Wipers may be fitted to the static mixer elements to enable relative movement between the static mixer elements and the surfaces of the elongate ultraviolet radiation sources to clean the surfaces of the sources without need to dismantle the apparatus.