Two types of molecular simulation techniques have been utilized to investigate surface modification effects on adsorption of acetone/water in mesoporous silicas with a hexagonal uniform pore structure: the NVT-ensemble Molecular Dynamics technique with the melt-quench algorithm for modeling a nonsilylated mesoporous silica (an 011 surface model) and a fully silylated mesoporous silica (an FS surface model), and the mu VT-ensemble Orientational-Bias Monte Carlo method for calculating adsorption isotherms. A good agreement was obtained between simulations and experiments for adsorption of pure acetone and water at 298 K on non-silylated and silylated mesoporous silicas. Equilibrium adsorption densities of the equi-fugacity mixture of acetone and water are calculated for the OH and FS surface models, by specifying either the gas or the liquid phase for the mixture. A considerably large separation factor, 7300, was obtained for the adsorption from a liquid mixture in the FS surface model, though the adsorption amount was rather small, which explains why the pervaporation through highly silylated mesoporous silica membranes was effective for separation of organic/water mixtures.