Abstract: The sand and silt fractions of soils derived from Tertiary basalt in northeastern Ireland exhibit unusually high cation-exchange capacities. Smectite was identified by X-ray powder diffraction in all size fractions and was selectively dissolved by treatment with 1 N HCl and 0.5 N NaOH. Structural formulae, based on chemical analysis of the dissolved material, suggest that the smectite is intermediate in composition between a di- and a trioctahedral mineral and that the octahedral cation occupancy, together with the numbers of Mg and Fe atoms per unit cell, decreases with decrease in particle size of the soil fraction examined. The mean formula for the smectite in the clay separates is: M+0.40(Si3.33Al0.67)(Al1.07Fe2+0.09 Fe3+0.41,Mg0.82)O10(OH)2. Similar analysis of smectite present in clay-size material separated from weathered basalt rock taken from the bases of the soil profiles showed that it had close to the full trioctahedral occupancy. It is suggested that the soil smectite is a weathering product of the trioctahedral smectite inherited from the weathered basalt.