Intrusion of water into a pore space of silicas hydrophobized with alkylsilanes is studied. The water porosimetry technique is introduced and construction of a water porosimeter is described. The effects of bonded alkyl chain length, bonding density, structure of alkylsilane, and average pore diameter of silica on the parameters of water intrusion are studied. Advancing and receding angles of water on chemically modified silica surfaces are estimated. It is shown that contact angles increase as bonding density and bonded chain length are increased. The correlation between the structure of a bonded alkylsilane monolayer and its hydrophobicity is discussed.