Penetration of salt solutions into cation exchange resins was used to measure the volume of a space within the resin phase (the inner space), which was usable for ion exchange in principle but not fully in practice. The average volume of the inner space per functional group was significantly dependent on the type and the concentration of the salt (250-450 Å 3) for a resin having the cross-linking degree of 8%, while being less dependent on the type and the concentration of the salt (160-240 Å 3) for a resin having the cross-linking degree of 12%. The salt concentration ratio of the solution filling the inner space to the outer solution decreased with a decrease in concentration of the outer salt solution due to the osmotic pressure, and decreased with an increase in cross-linking degree due to the smaller volume and the higher independency of the inner space.