The paper concerns the use of Donnan dialysis for the modification of ionic composition of brackish solution before its desalination. The reason for including this kind of pretreatment in the system of water desalination is the need of reducing or eliminating the precipitation of sediments on the membrane surface. This precipitation, known as fouling, may lead to deterioration of the desalination efficiency and to the membrane damage. The authors used two types of ion exchange membranes: the anion-exchange membrane Neosepta AFN and cation-exchange membrane Selemion CMV. The use of the AEM membrane allowed to replace with chlorides up to 88% of sulfates and up to 66% of bicarbonates present in the raw solution. While using the CEM membrane, it was possible to exchange for sodium cations up to 89% of calcium and up to 90% of magnesium. Brackish solutions after the process contained more soluble salts than the raw brackish solution, which should have advantageous influence on the desalination process itself. The authors analyzed factors influencing the process efficiency -concentration of the receiver, volume ratio of the streams separated by the membrane and the composition of the receiver used in Donnan dialysis.