In the paper the authors assessed usefulness of Donnan dialysis with cation-exchange membranes for the removal of troublesome calcium and magnesium cations before electrodialytic water desalination. The rate and efficiency of cation exchange were examined with the use of two types of cation-exchange membranes: Neosepta CMX and Selemion CMV. The authors analyzed the influence of some process parameters (salt concentration in the receiver and stream volume ratio) on the ion exchange. It was observed that both membranes examined assured similar effect of the Ca2+ and Mg2+ removal, i.e. from 80 to 88% of removal, with the NaCl concentration in the receiver 200 mmol/dm(3) and the stream volume ratio 4: 1. However, ionic fluxes for the Selemion CMV membrane were higher by about 30% than those for the Neosepta CMX membrane - and that significantly reduces the time of the process. In the second part of the paper, the authors compared the results of electrodialysis of a raw two-component solution and of a solution after cation exchange. It was concluded that due to the change of ionic composition of the solution, electrodialysis can be conducted at a higher current density, and the salt removal rate is more than 20% higher (the salt fluxes from the raw solution and from the solution after Donnan dialysis are, respectively, 0.421 and 0.512 eq /m(2) [h, at a current density of 30 A/m(2)). The authors also observed a positive effect of the changed ionic composition of the solution on the energy consumption for the ion transport: the removal of a given amount of salt requires up to three times less energy than in the case of the raw solution.