MCM-41 silicates prepared in the presence of octyltrimethylammonium bromide either by a conventional method or by postsynthesis hydrothermal treatment were characterized by nitrogen adsorption in a wide range of relative pressure from 10(-6) to 1. Hydrothermally restructured samples were found to have lower BET surface areas, lower external surface areas and thicker silica walls than the non-treated sample. More importantly, in addition to their characteristic mesopores (ca. 3 nm), they were shown to have considerable amounts of micropores. The relative amount of micropores and mesopores was shown to be dependent on the treatment conditions. Thus, it is demonstrated that the postsynthesis hydrothermal restructuring is a convenient synthesis route to MCM-41 silicates with bimodal pore size distribution involving controllable amounts of microporosity.