An MCM-48 membrane was prepared on a porous alpha -alumina support by hydrothermal synthesis. The silylation with trimethylsilane and triethylsilane was used to enhance hydrothermal stability and hydrophobicity of the MCM-48 membrane. The studies of X-ray diffraction, Fourier transform infrared (FTIR), nitrogen adsorption, and water adsorption were carried out using an MCM-48 powder to characterize the pore surface of MCM-48. The FTIR study indicates that the silyl group was substituted for the Si-OH group on the pore surface of MCM-48. The pore sizes of the nonsilylated and the silylated MCM-48 were 2.4 (nonsilylated), 1.9 (trimethylsilylated), and 1.8 nm (triethylsilylated). The water adsorption study showed that the pore surface of MCM-48 significantly changed to hydrophobic by silylation. The nonsilylated and the trimethylsilylated MCM-48 membranes were used for the separation of an ethanol/water mixture. The separation factor was 16 through the silylated membrane, although that of the nonsilylated membrane was 0.3, indicating that the pore of the silylated membrane hardly adsorbed water. Furthermore, the hydrothermal stability of the MCM-48 membrane was improved by the silylation.