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The influence of the interlayer cations, sonication and grinding on surface area of the vermiculite from Santa Olalla has been studied. For untreated samples the surface area increases with the size of the exchanged cation and only the samples with the largest cations (NH+ and K+) show some microporosity. Sonication produces an increase of the surface area but the amount of micropores remains almost unchanged. Grinding leads to a significant increase of the surface area and of the microporosity. In the ground samples the percentage of micropores contributing to the surface area is influenced by the interlayer cation and the extent of N2 penetration by the water kept in the sample at a given degassing temperature. From transmission electron microscopy, thermogravimetry and XRD (d00l diffraction peaks) it can be concluded that grinding produces imperfections in crystal ordering, some alteration in the crystal edges, and a significant release of loosely bound water at temperatures up to 450 8C. These effects are responsible for the high surface area and microporosity measured by the N2 adsorption.