A rotating disc diluter (1) has a rotatable rotary element (4) which carries surface-accessible transfer volumes (9, 10) which along their common path of movement alternately glide over feed and discharge ports (11-18) for an undiluted fluid flow on the one hand and for a dilution fluid flow on the other hand. For the simple widening of the usable dilution rate range, the rotary element (4) has at least two rows of transfer volumes (9, 10) on different paths of movement, the associated feed ports (11-14) of which for the undiluted fluid flow and/or for the dilution fluid flow can be separately controlled.