An electrodeionization post-reverse osmosis (RO/EDI) system was developed for the continuous production of high-purity water without the use of chemicals. The study was carried out with a laboratory pilot. The ion-exchange membrane stack has eight cell-pairs of which four dilute compartments were filled with mixed-bed ion-exchange resins. The feed is tap water with a conductivity of about 400 mus/cm, and the RO permeate with a conductivity less than 20 mus/cm. The EDI was achieved with a productivity of 201/h and a resistivity of 12-18 M Omega -cm. In addition, the V-I and pH-I characteristics of the EDI process were studied. It indicates that there are differences between the ED and the EDI in the concentration polarization.