The microporosity of a series of Al-pillared clays prepared with different Al/clay ratios has been studied. Nitrogen adsorption data obtained from very low relative pressures has been used to obtain a thorough characterization of the microporosity of the intercalated solids. Dubinin-Radushkevich plots, adsorption potential distributions, and micropore size distributions obtained with the Horvath-Kawazoe approach confirmed the existence of two sizes of micropores. The Al/clay ratio played a determinant role in the properties of the samples obtained. For the lower values, the intercalation process was not successful and a low microporosity was obtained. For 30 and 90 mM Al/g of clay a basal spacing of 1.84 nm was found in accordance with a great microporosity increase. For the higher Al/clay ratio (180 mM), an excess of Al2O3 was introduced between the clay layers, and in spite of a large basal spacing both the micropore volume and the micropore area decreased.