Low hydrothermal stability in hot water and aqueous solutions was a critical problem for many applications of the mesoporous molecular sieve MCM-41, compared with conventional zeolites such as Y and ZSM-5. The hydrothermal stability of MCM-41 has been found to improve remarkably by using various salts such as sodium chloride, potassium chloride, sodium acetate, and ethylenediaminetetraacetic acid tetrasodium salt during the hydrothermal crystallization process performed at 370 K. High-quality MCM-41 samples obtained by using the salt effect indicated negligible structural losses as judged by X-ray diffraction during heating for 12 h in boiling water.