Abstract: Modifications of the external surface area and the two types of microporosity of sepiolite (structural microporosity and inter-fiber porosity) were examined as a function of the temperature of a vacuum thermal treatment to 500°C. The methods used included: reciprocal thermal analysis, N2 and Ar low-temperature adsorption microcalorimetry, gas adsorption volumetry (for N2, Ar, and Kr at 77 K and CO2 at 273 and 293 K), water-vapor adsorption gravimetry, and immersion microcalorimetry into liquid water at 303 K. If the sample was not heated > 100°C only 20% of the structural microporosity was available to N2, whereas 52% was available to CO2 at 293 K. In both experiments, the channels filled at very low relative pressures. At > 350°C, the structure transformed to anhydrous sepiolite, which showed no structural microporosity. The inter-fiber microporosity decreased from 0.031 to 0.025 cm3/g (as seen with N2), and the external specific surface area decreased from 120 to 48 m2/g. The water adsorption isotherms showed a lower and lower affinity of the external surface of fibers for water as the temperature of thermal treatment increased. The thickness of the bound water on the external surface was estimated to be ≤3.5 monolayers, i.e., less than 10 Å.