Nitrogen and water adsorption properties on the porous silica glass surfaces modified by different concentration of trimethylsilyl (TMS) groups were studied. The specific surface areas and parameter C values in the BET equation estimated from nitrogen adsorption decreased with increase in the surface concentration of the TMS groups and became constant values above 1.0 TMS group per nm(2). The water adsorption property suggested that a highly silylated sample still represented hydrophilic character in initial adsorption process. Water molecules interact with the residual hydroxyl groups existing under the umbrellas of the TMS groups and form clusters. Above the concentration of 1.0 TMS group per nm(2) the capillary condensation of water vapor did not occur. The TMS groups interrupt to build a two-dimensional network of each cluster. The formation of multilayer adsorption of water molecules depends on the magnitude of hydrophilic patch areas which are surrounded by TMS groups.