Microporosity and connectivity of ordered mesopores of SEA-15 silica were studied using nitrogen adsorption and novel methods based on selective pore blocking via organosilane modification, and on the imaging of inverse platinum replica of ordered mesoporous structure. It was found that SEA-15 exhibits a relation between the pore size, pore volume and specific surface area which is significantly different from that for cylindrical or hexagonal pores, which suggests that the SEA-15 structure is more complex than an array of hexagonally ordered channels, even if they are corrugated. Nitrogen and argon adsorption measurements provided evidence that large mesopores are accompanied by a certain amount of significantly smaller pores (of the size below about 3.4 nm) with a broad distribution primarily in the micropore/small-mesopore range. The modification of SEA-15 via chemical bonding of small trimethylsilyl ligands partially blocked the complementary pores, and the bonding of larger octyldimethylsilyl groups made them essentially: fully inaccessible to nitrogen molecules, which manifested itself in dramatic changes in the relation between the pore size, pore volume, and specific surface area. After dissolution of the SEA-15 framework, platinum wires grown inside the porous structure formed bundles, as seen from transmission electron microscopy. These results provided strong and unambiguous evidence that large ordered mesopores of SEA-15 are accompanied by much smaller disordered pores and that an appreciable fraction of the latter is located in the pore walls, providing connectivity between the ordered large-pore channels. The complementary pores are suggested to form as'. result of penetration of poly(ethylene oxide) chains of the triblock copolymer template within the silica framework of as-synthesized SBA-15. We also studied thermal stability of SEA-15 structure and its complementary porosity. As inferred from nitrogen adsorption data, the complementary porosity was retained to a significant extent even after calcination at 1173 K, but most likely completely disappeared at 1273 K. The heat treatment was accompanied not only by a significant decrease in the specific surface area and pore volume but also by narrowing the pore size distribution at temperatures up to 1173 K. Thus, we were able to demonstrate for the first time that the SBA-15 sample with nitrogen adsorption properties similar to those of MCM-41 can be obtained via calcination at 1273 K, although the pore volume and specific surface area of such SEA-15 material is relatively low.