A natural saponite was acid activated at room temperature or 90 degrees C with different acid/clay ratios and the products were characterized by powder X-ray diffraction, infrared spectroscopy and thermogravimetry. The leaching of Mg from the octahedral sheets is enhanced by an increase in the acid/clay ratio and by an increase in temperature of activation. Textural properties are reported, and it appears that they are strongly correlated to the presence of a non-crystalline silica phase which is formed during the acid activation process. The desorption of cyclohexylamine indicates that for samples activated at 90 degrees C, the number of acid sites in the acid-activated saponites decreases following severe acid treatment. Infrared spectroscopy of adsorbed pyridine on samples after calcination at 500 degrees C suggests that acid activation at 90 degrees C produces a single type of Bronsted site but two types of Lewis sites whereas activation at room temperature results in only one type of Lewis site in addition to a Bronsted site. The two Lewis sites are suggested to originate from residual Al in the clay structure and to Al exsolved from the layers during activation. The dehydration of pentan-1-ol has been used as a further probe to measure acidity by monitoring the degree of conversion and selectivity for the different samples.