The equilibrium relation between mixed cation and anion exchange resins and a dilute salt in water are investigated. Considering the equilibrim characteristic values of each ion exchange and also the mixed-volume fraction of both resins, equilibrium equations are derived on the basis of both cation and anion exchange reactions and the neutralization reaction between H+ and OH- eluted from the resins. When both resins are mixed in equal equivalence for ion exchange, the equilibrium relation becomes a rectangular isotherm and is not affected by a concentration of a salt, ionic valence of species composing the salt, or the equilibrium constants of both ion exchanges. When mixed in unequal equivalence, the deionization equilibrium is influenced by the above factors. Especially, the effect of the mixed volume fraction on the equilibrim relationships depends greatly on the valence of ions composing the salt. The experimental results using aqueous NaCl solution showed good agreement with the calculated ones.