The thickness, length, width, area and perimeter of 575 particles from 16 aqueously dispersed samples of a variety of interstratified clays, smectites and illite have been recorded using TEM techniques. Complete dispersion of the clay material was achieved by saturating the clay with either Na+ or Li+, removal of excess ions by dialysis, and isolation of the <0·1 or <0·2 µm fraction by centrifugation. The samples have mean maximum dimensions of 1900 to 90 nm and the dispersed system can be considered as colloidal in nature. The mean thickness of the clays is about 1 nm for smectites, corresponding to that of elementary 2:1 silicate layers, from 1·9 to 4·9 nm for the interstratified clays, and 9 nm for illite. From these data the volume, total surface area and other parameters have been calculated and compared with independent determinations of surface area and CEC. The total surface area by TEM, assuming a density of 2·6 g/cm3, varies from ∼675 m2/g for smectites to 86 m2/g for illite, and is inversely proportional to the mean particle thickness. The charge density of monovalent cation exchange sites on the surface of the particles as determined for nine of the samples varies from 0·54 to 1·16 nm2/site. The particle-thickness distribution data can be used to calculate interstratified XRD layer-sequence probabilities and composition parameters, and agree with XRD data for interstratified clay with <60% illite layers. The thickness data also provide a rationale for the interpretation of TEM lattice-fringe images. Relationships between the particle area, length, thickness and volume are shown to be potentially useful in assessing the mechanism(s) of crystal growth of these extremely small phyllosilicate particles.