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Portuguese srnectites and palygorskitestwere treated with 1.0, 5.0, 7.0 and 9.0 N solutions of HCl for one hour under reflux. The solids obtained were characterised by their loss weight evaluation, X-ray diffraction, chemical analyses, surface area, texture and TEM. The results indicate that the mechanism of the activation shows a point of inflection at a certain acid concentration, which is of 7.0 N for palygorskite rich samples, and 5.0 N for smectite rich samples. We never assist to a complete destruction of the clay minerals structure. The proper-ties of high acid attacked clays are very similar to those of the original material or the lower attacked one, what can be interpreted as<< passivation» of the clay, resulting from the silica created in acid activation, that coats the silicate imped-ing further attacks.