An apparatus and a method for preparing dialysis fluid for use in extracorporeal blood treatment, wherein the apparatus includes a main line for the supply of water in the course of which each of an acid fluid and a basic fluid is fed at a particular dosage, wherein the dosage is set by a control and regulating unit as a function of at least one chemical and/or physical parameter of the water-fluid mixture and the parameter is detected by a measuring instrument wherein a first measuring instrument is arranged at a section of the main line located downstream of each of the feeding point for the acid fluid and the feeding point for the basic fluid and the control and regulating unit at least temporarily controls the feeding of the acid and basic fluids such that over a predetermined period of time or a predetermined interval only one from among the acid and basic fluids is fed into the main line.
