An oriented gel of homoionic montmorillonite was supported on a porous filter in a metal environmental chamber fitted with beryllium windows for the transmission of X rays. Beneath the filter was a shallow reservoir connected to the outside atmosphere. Solution was expressed from the gel into the reservoir by admitting nitrogen gas to the environmental chamber at successively higher pressures. At each pressure the expressed solution in the reservoir was allowed to equilibrate through the filter with the gel and then the distance between the superimposed layers of montmorillonite in the gel was measured by X ray diffraction. The swelling pressure of the montmorillonite equals the applied pressure at equilibrium. Thus, the relation between the swelling pressure and interlayer distance of the montmorillonite was determined when it was saturated with different exchangeable cations and equilibrated with electrolyte solutions of different concentration. The experimental results showed that, at relatively low concentrations of electrolyte, neither the species of exchangeable cation nor the electrolyte concentration had any effect on the relation between the swelling pressure and the interlayer distance. However, at relatively high concentrations of electrolyte, both of these factors affected this relation. Since the effect of the electrolyte concentration was not described quantitatively by electric double-layer theory, it was assumed that this theory was not applicable and that the added electrolyte reduced swelling by disrupting the hydration shells surrounding the montmorillonite particles.