Three 'pure' illites were studied. Two, extracted from soils, were fine grained, and the other coarse, extracted from a shale. The average particle size of each clay was determined from electron micrographs and a specific surface area calculated. For the two soil illites any difference in the surface area deduced from water vapour sorption as compared with nitrogen sorption or by direct calculation, was consistent with the purity of the clay as determined by X-ray diffraction. For the shale, there was a large discrepancy, which is attributed to this illite having a greater surface density of charge. For the two soil illites, the exchange of calcium or magnesium for sodium, over the ESP range 0-12 and the calcium-magnesium exchange isotherms were determined. The exchange of sodium followed the same Gapon-type equation for both clays, and was independent of whether calcium or magnesium was the dominant cation. The calcium-magnesium isotherms were also symmetrical. No evidence was therefore found for calcium ions being more strongly held at exchange sites than magnesium ions. It was shown that in the solutions used to equilibrate the clays, which were 1 mM in divalent cations, diffuse double layers were not formed.