Due to its channels of molecular dimensions and a high specific surface area, sepiolite has many industrial applications which require high resistance to thermal effects in addition to a large surface area. On heating, sorbed water molecules are removed causing changes in the pore size distribution. In this study, the effects of thermal treatment on the pore structure of sepiolite and the acid-treated sepiolite samples were investigated. The solid density of sepiolite, measured by a He displacement technique, was 2.08 g cm(-3) and total porosity was similar to 0.58. Both of these values showed an increase at 100 degrees C, then decreased with further temperature increase due to crystal deformation and channel plugging which occurred at elevated temperatures. The BET surface area of the original sepiolite was 148 m(2) g(-1), and increased to 263 m(2) g(-1) at 100 degrees C and then started to decrease. Approximately 16% of the total volume was in the micropores at 100 degrees C. The acid pre-activation caused restrictions in possible crystal deformation during thermal treatment. The micropore volume increased to 20% and BET surface area reached values >500 m2 g(-1) for the acid-treated samples.