A solid ion-exchange electrolyte has successfully been used in the electrochemical deoxygenation of low-conductivity water. A packed bed three-dimensional cathode was mixed with ion-exchange resin. Oxygen-rich low-conductivity water was passed through the three-dimensional cathode and the dissolved oxygen was reduced to water. A sufficient ionic conductivity in the three-dimensional electrode was obtained for both distilled water and tap water by small-size strong cation exchange resins in the H+ and Ca2+ forms, respectively. Deoxygenation efficiencies of over 99.9% were achieved.