Two types of molecular simulation techniques have been utilized to investigate surface modification effects on adsorption of methanol/water in mesoporous silicas with hexagonal uniform pore structure: the NVT-ensemble Molecular Dynamics technique with the melt-quench algorithm for modeling a non-silylated mesoporous silica (an OH surface pore model) and a fully silylated mesoporous silica (an FS surface pore model), and the mu VT-ensemble Orientational-Bias Monte Carlo method for calculating adsorption isotherms. In the OH surface pore, the adsorption isotherms of pure gases at 298 K show a stepped curve for methanol and a catastrophically increasing curve for water, the latter of which is characteristic of the condensation mechanism. The simulation isotherms for each gas are in good agreement with the experiments. In the FS surface pore, water does not adsorb at elevated pressures while methanol shows an adsorption isotherm representing the condensation mechanism, which indicates that the surface silylation weakens the adsorption affinity of methanol as well as water. Equilibrium adsorption densities have been calculated at 333 K for an equi-relative-pressure mixture (a mixture in which each component has the same relative pressure, i.e., the partial pressure divided by the saturation pressure of each component). It is noted that, in the FS surface pore, water are substantially adsorbed along with methanol, showing an isotherm representing the condensation mechanism. The local density profiles of the two components indicate that methanol is preferentially localized near the pore surface due to the hydrophobic interactions between the CH3 group and the pore wall, while water and methanol form a fluid phase with a homogeneous composition.