Orientated flakes of dry Na-montmorillonite were brought into equilibrium successively with relative vapour pressures of 0.92 and 0.985 and the moisture contents, (001) spacings, and swelling measured, the latter with a travelling microscope. Over saturated water vapour the clay continued to swell with time, in contrast to Ca-montmorillonite. Hysteresis in swelling was only observed at values of p/po > 0.985. The further expansion of flakes when immersed in solutions, N in Na+, of sodium chloride with dilute buffer added (from pH 4.4 to 10) or sodium hydroxide or chloride plus neutral sodium pyrophosphate was determined. The Na+ concentration was then reduced and the corresponding swelling measured, until the flake dispersed. Similar experiments were made on orientated flakes prepared from Na-montmorillonite to which 1.5% cetyltrimethylarnmonium bromide (CTAB) had been added, as well as from Na-montmorillonite which had been washed with lithium chloride and heated before reconversion to the Na form. The expansion of Li-vermiculite crystals in lithium chloride solutions was also determined. In solutions N in Na+ the swelling of Na-montmorillonite flakes was independent of pH, but for Na+ < N/2 the swelling increased sharply above pH 8.0. The swelling of Na-clay + CTAB was much less than untreated clay and was independent of pH. Na-montmorillonite which had been lithium-treated at 95�C gave a swelling pattern at pH 4.4 similar to that of Na-clay + CTAB, whereas in sodium hydroxide the pattern was similar to that of untreated clay. The specific effect of the pyrophosphate anion on the swelling of the CTAB-treated clay was slight. There was marked hysteresis in the swelling of Na-montmorillonite with respect to salt concentration, whereas the swelling of Li-vermiculite was almost reversible. Adding CTAB inhibited the intercrystalline swelling of Na-montmorillonite, the CTA+ ions forming Stern layers on the external surfaces of the crystals. A similar effect was apparently produced in the acetate buffer by aluminium ions released during the lithium treatment. The increase in the swelling of the untreated clay with pH is consistent with the removal of aluminium ions from the external surfaces of the crystals. There must be residual attractive forces between the crystal at high pH to account for the stability of the clay in dilute salt solutions. The edge to face forces linking the silicate sheets together appear to be constant above pH 4.0. Neutral sodium pyrophosphate disperses the clay at Na+ concentrations of < N Na+ by removing aluminium ions and neutralizing positive edge charges. The montmorillonite crystals are considered to be linked mainly edge to edge in a tactoid. The bands observed in thin sections of expanded gels, using polarized light, may be due to a periodicity in the stacking of the silicate sheets forming the crystals.