Siliceous MCM-48 spheres are tailored with silane coupling agents, n-octyldimethylchlorosilane (C8) and (3-aminopropyl)triethoxysilane (APTS). The changes in adsorption features of water and acetaldehyde with the modifications are elucidated by aid of adsorption calorimetry. The MCM-48 surface exhibits a hydrophobic feature after the C8 treatment, and the hydrophobicity is retained by repetition of water sorption, although the surface of crude MCM-48 easily changes to hydrophilic by water adsorption. The APTS treatment drastically increases an adsorptivity of acetaldehyde vapor. The adsorbed amount of acetaldehyde is saturated at ca. 5 mmol/g for crude MCM-48; however, that for the APTS-treated sample is three times higher at equilibrium pressure (P-e) of ca. 6 kPa and monotonically increases at P-e > 6 kPa. Differential heats of adsorption during the mesopore filling of acetaldehyde are 53-70 kJ/mol, which is much higher than the heat of liquefaction (27.2 kJ/mol) of acetaldehyde, suggesting that chemical reactions of adsorbed acetaldehyde take place on the surface.