In this study, the cation exchange capacity (CEC) and leached exchangeable cations (LC) of montmorillonite purified irons bentonite produced in the Tsukinuno bentonite mine, Yamagata, Japan, were measured, and the exchangeable cations in the interlayer of the montmorillonite were discussed. A montmorillonite, in which the soluble minerals were completely removed, was prepared. Kunipia-F and Kunipia-P, for which both bentonites originally contain approximately 100 wt.% montmorillonite, were used as the initial material. All of the measurements were carried out in a N2 atmosphere-controlled glove-box. The CEC values of montmorillonites for both bentonites (100-110 rneq/100g) were similar to data conventionally reported, and the sum of LC was also approximately in good agreement with the CEC values. The share of Na + in the interlayer of montmorillonite calculated from the LC was about 3/4 of the sum of the LC (≈ CEC), and Mg2+ and Ca 2+ occupied about 7 and 19 %, respectively. Although montmorillonite in bentonite produced in the Tsukinuno bentonite mine is known as a Na type, the sum of Ca2+ and Mg2+ occupied about 26 % of all exchangeable cations in the interlayer. Based on these data, the ion exchange reaction constant between Na+ and H+ in the interlayer of montmorillonite was calculated to be -0.07. This is nearly 2 orders of magnitude lower than data that are usually adopted.