Particle-size analysis (obtained by Galai CIS 1) was used to determine the following statistical parameters of aqueous suspensions of Na-, Ca- and Mn-montmorillonite: the size distributions, the diameters of the largest and of the median and mean particles, and the percentage of particles with diameters <1.5 μm. Dilution or shaking had almost no effect on the particle-size distribution curves and statistical parameters of Ca- and Mn-montmorillonites but the curves and the statistical parameters of Na-montmorillonite were very much affected by these treatments. The median and mean diameters of Na-montmorillonite range from 0.8 to 10.5 μm and 0.8 to 11.6 μm, respectively, the median and mean diameters of Ca-montmorillonite range from 1.5 to 3.6 μm and 2.12 to 4.2 μm, respectively, and those for Mn-montmorillonite from 1.2 to 2.3 μm and 1.5 to 2.5 μm, respectively. The presence of large particles of Na-montmorillonite was attributed to the extensive swelling of this clay in aqueous suspensions by osmotic water adsorption. The median and mean diameters of aged Na-montmorillonite suspensions indicate that swelling increases with dilution. The swelling of Ca- and Mn-montmorillonite, on the other hand, is limited and their particle size does not increase with dilution.