We describe a simple method of controlling the channel diameter of the mesoporous molecular sieve MCM-41 in the 26.1–36.5 Å range and the wall thickness in the 13.4–26.8 Å range while using the same gel mixture. This is achieved by varying the synthesis temperature in the 70–200°C range and/or reaction times in the 0.5–96 h range. The unit cell parameter, channel diameter, thickness of the channel wall, surface area, degree of polymerization and grain morphology were monitored by X-ray diffraction, N ₂ adsorption, ²⁹ Si magic-angle-spinning NMR and transmission electron microscopy. MCM-41 with wider and thicker-walled channels and higher degree of polymerization is prepared at higher temperatures and at longer reaction times. Thick-wall MCM-41 has higher thermal stability but lower surface area. The material with the thickest channel wall ever reported (26.8 Å) can withstand calcination at nearly 1000°C with little structural damage. We suggest a mechanism for the increase of wall thickness and channel diameter. Fascinating morphological features involving sealed silicate ‘tubes’ and ‘vesicles’ up to 1200 Å in diameter are observed.