Thicker-walled mesoporous MCM-41 silicas with improved thermal and hydrothermal stability were prepared via secondary crystallization (in which calcined MCM-41 was used as silica source) or by using long crystallization times. Unusually, the thicker pore walls of the as-synthesized (surfactant containing) forms of these Si-MCM-41 materials are primarily made up of fully condensed silica units. Q(4)/Q(3) ratios as high as 3.4 or 4.5 are obtained for two-step (48 h each) crystallization or 96 h crystallization at 150 degrees C, respectively. A diffusion-controlled mechanism in which silicate units and surfactant molecules are continually added onto the "seed" crystals (during secondary recrystallization) or to the growing surfactant/silicate aggregate (for long crystallization time) is proposed to explain the pore wall thickening and higher silica condensation.