SummaryStarting with orientated flakes of dry Ca‐montmorillonite (Wyoming bentonite) about 0·2 mm. thick, the swelling of the flakes when immersed in 2 M CaCl2was measured with a travelling microscope. The further expansion when the flakes were brought into equilibrium with successively more dilute CaCl2solutions was also measured. The swelling of the flakes in 2 M CaCl2was equal to that of flakes equilibrated with saturated water vapour. As the CaCl2concentration was reduced the flakes swelled progressively, with negligible increase in the (001) spacing. This swelling was independent of the pH of the CaCl2solutions and could not be reversed by increasing the concentration of CaCl2. Flakes prepared from clay suspensions to which 1 per cent. of CTAB had been added did not swell in dilute CaCl2solutions.Measurements were made of the swelling of flakes under an applied suction. Suction reduced the swelling as compared with free solution, and the swelling was less sensitive to CaCl2concentration. There was some hysteresis in swelling between the wetting and drying cycles.The results are consistent with the swollen Ca‐montmorillonite being in the form of a tactoid, the arms of the tactoid consisting of the clay crystals aggregated end to end. Swelling is due to the development of double layers between crystals in the pores of the tactoid. The linkages at the joints of the tactoid arise because either there is perfect orientation between the crystals at these points or there is a small number of complex ions (≤ I m.e./100 g. clay) other than Ca‐ions present on the external surfaces of the crystals. CTA+ions, being strongly absorbed by the clay, increase the number of such linkages, so inhibiting double‐layer swelling. Whether or not Ca‐montmorillonites in general exhibit double‐layer swelling will depend critically on how many ions other than Ca‐ions are pre