The enthalpy changes, Delta H, in suspensions of Li-, Na-, and K- montmorillonite were determined as functions of the molal concentrations of LiCl, NaCl, and KCI, respectively, by microcalorimetry. These changes were compared with the corresponding changes in interlayer distance, lambda, as determined by X-ray diffraction. It was found that Delta H values accompanying the collapse of the montmorillonite layers from lambda > 14 nm to lambda congruent to 1.0 nm (or 0.6 nm for K-montmorillonite) were essentially negligible; however, Delta H values accompanying the collapse of layers of Li- and Na-montmorillonite from lambda congruent to 1.0 nm (three-water-layer hydrate) to lambda congruent to 0.6 nm (two-water-layer hydrate), and the collapse of layers of K-montmorillonite from lambda congruent to 0.6 nm (two-water-layer hydrate) to lambda congruent to 0.3 nm (one-water-layer hydrate), were not negligible. No additional collapse of Li- and Na-montmorillonite layers occurred after lambda congruent to 0.6 nm, and for K-montmorillonite layers after lambda congruent to 0.3 nm, but Delta H continued to change with increasing salt addition. This change was attributed to the penetration of electrolyte into the interlayer space. A notable observation was that the value of Delta H was positive for Li-montmorillonite, but negative for Na- and K-montmorillonite. These observations were interpreted to mean that Delta H changes in the montmorillonite suspension were largely attributable to reorganization of the interlayer space by the added electrolytes.