The structure of pores formed in the alumina- or chromia-pillared montmorillonite (PMT) interlayer was investigated by analyzing the porous solid itself and the negative carbon replica formed in the pores of the parent material. The nitrogen adsorption on these materials allowed us to estimate the pillar structure and the density of pillar sites in the montmorillonite interlayers, The amount of Al-13 polycation in an exchanging solution governed the interpillar distance in the pillared clay gallery without changing the diameters of the pillar (ca. 0.6 nm), The aperture size of Al-PMT was 0.8 nm in height and 0.5-0.9 nm in width depending on Al content, On the other hand, the aperture size of Cr-PMT was 0.5 nm in height and 0.8 nm in width, and the diameter of a pillar was estimated to be ca. 0.4 nm.