Highly ordered MCM-41 silicas were synthesized using a new procedure and their structures were thoroughly characterized. Using the new synthesis method and the previously reported similar one, the pore diameter of MCM-41 can be tailored from 3.1 to 4.9 mn with about 0.35 nm increments and reproducibility usually better than +/-0.1 nm using surfactants of different chain length. The surfactant content and structural parameters of MCM-41 synthesized using the same single surfactant or surfactant mixture were highly similar for different silica:surfactant molar ratios in the synthesis mixture. Moreover, the silica:surfactant molar ratio in as-synthesized MCM-41 was quite independent of the chain length of the surfactant used and approximately equal to 8. This may be related to the well-defined synthesis conditions imposed by the pH adjustment procedure employed. The results indicate that the silica: surfactant ratios in starting mixtures suitable for the synthesis of high-quality MCM-41 under the present synthesis conditions should not exceed about 8 to avoid contamination with amorphous silica. (C) 2001 Elsevier Science B.V. All rights reserved.