Mesoporous silica, SBA-2, has been investigated by using high-resolution transmission electron microscopy supported by computer image simulations. The complete pore structure connecting the discrete supercages of which the silica is composed has been determined. In addition, unexpected well-defined cubic-hexagonal (polytypic) intergrowths have been uncovered, involving a hitherto unknown mesoporous structure that we designate STAC-1, It was found that both SBA-2 and STAC-1 contain a two-dimensional pore system and that the symmetry of the SBA-2 structure must be lower than that (space group P6(3)/mmc) determined previously on the basis of X-ray powder diffraction methods. Polytypic irregularities in these mesoporous materials indicate that, similar to microporous zeolitic systems, a range of solids with structures intermediate between end members SBA-2 and STAC-1 might be prepared.