Sepiolite is a high surface area clay mineral containing channels of molecular dimensions. In industrial applications as an adsorbent or catalyst support, the mineral should have thermal stability. In this study, the effect of heat treatment on sepiolite structure was investigated. Thermal decomposition of raw sepiolite and acid pre-activated sepiolite samples were carried out with a thermogravimetric analyser. Approximately 24% weight loss was observed till 900 degrees C. This value decreased to 14% for acid pre-treated samples. The surface area value of the original sepiolite was 148 m(2) g(-1), and it increased to 263 m(2) g(-1) at 100 degrees C. Further increase in temperature caused channel plugging and crystal structure deformation, as a result the surface area values showed a decrease with temperature. On the other hand, acid pre-treatment caused a change in pore size distribution by increasing the surface area 2.5 times. Acid pre-treatment also restricted the possible crystal structure deformations during temperature changes. While the surface area of the original mineral which was heated up to 900 degrees C was 60 m(2) g(-1), the corresponding value of the acid pretreated sepiolite was 360 m(2) g(-1).