The stability of silicate MCM-41 in aqueous solutions for up to 12 days has been dramatically improved by silylation with hexamethyldisilazane (HMDS). Mesoporous molecular sieves including MCM-41 show promise for application as selective media for separations of large molecules such as proteins, which are important in the food and pharmaceutical industries. However, many such applications involve aqueous solutions in which MCM-41 has limited stability. Thus, hexamethyldisilazane has been used to create a hydrophobic surface coating on both flat silica and MCM-41 in order to improve their stability in aqueous solutions. This coating was shown to persist on the flat surfaces after aging in water for up to 7 days, using contact angle measurements and atomic force microscopy, unlike a coating applied using trichloromethylsilane which was removed by water. The HMDS-coated MCM-41 showed much greater structural stability than uncoated MCM-41 in both water and an aqueous potassium phosphate buffer solution of pH 6 for extended periods.