For deionization by a mixed bed, a mass transfer model is presented on the basis of both cation and anion exchange reactions and neutralization reaction. Cation and anion exchange characteristics and mixed fraction of the two resins are considered in the model. Theoretical ionic breakthrough curves of the model equations were obtained by using a finite difference numerical method. Effects of salt, concentration, now rate of feed solution, bed height and mixed fraction of both resins on the breakthrough curves were examined theoretically and experimentally. The calculated breakthrough curves agreed with experimental data under various conditions. This result suggests that the proposed model can be applied to the deionization process in a mixed bed. The relation between the diffusion resistances of transfer ions and the operational conditions was also studied using a numerical simulation.