The processes of dehydration and rehydration appear quite different for natural Ca-montmorillonite (Ca-MMT, for short) and its sodium saturated form (Na-MMT). During dehydration the d₀₀₁ values of Ca-MMT sample drop continuously from 15.36 Å to 10.20 Å with increasing temperature of dehydration (from 25 to 300 °C), but the relation breaks down at 200 °C, above which the position remains equal to 10.2 Å. The mass loss Δm_200° corresponds to 9.5 water molecules per one interlayer cation. In contrast to Ca-MMT, the dehydration process of Na-MMT can be divided into three stages: (1) At temperatures ≤ 70 °C there is a small shift of d₀₀₁ from 12.90 Å to 12.48 Å; (2) At temperatures between 80 and 110 °C, there is a significant shift of the d₀₀₁ value to 10.95 Å; (3) Collapsed Na-MMT structure (10.2 Å) appears at above 150 °C. The mass loss Δm_150° corresponds to 4.9 water molecules per one interlayer cation. In the rehydration process of both Ca-MMT and Na-MMT two stages can be recognized. The first corresponds to temperature ranges from 25 to 200 °C (Ca-MMT) and from 25 to 150 °C (Na-MMT); there the rehydration ability drops to 61 % for Ca-MMT and 88 % for Na-MMT, however no significant collapse of the interlayer of rehydrated samples is observed. The second stage corresponds to temperatures between 200 °C (150 °C for Na-MMT) and 500 °C, where the rehydration ability drops significantly and so do the d₀₀₁ values.