Aqueous dispersions of homoionic sodium montmorillonite have been studied under compressive and decompressive conditions in order to determine the internal pressure of the system as a function of the clay and electrolyte concentrations. Complementary measurements were also made of the electrokinetic properties of the montmorillonite particles. Compression of a sodium montmorillonite dispersion from ca. 2 % to ca. 65 % w/w gave a continuous curve of pressure against distance of plate separation up to pressures greater than 100 atmospheres. On subsequent decompression, however, a different curve was obtained and a hysteresis effect was found to occur in the pressure against distance curves. It is considered that the initial curve represents the behaviour of a disordered clay system, i.e., the initial gel, whereas the decompression curve represents the behaviour of a system in which the plates have become ordered into a parallel array. The elastic modulus of the latter system was determined. The results strongly suggest that the gel properties of montmorillonite dispersions are the consequence of long range electrostatic interactions.