The intrusion of water into the pores of hydrophobic materials is investigated by means of a dynamic flow system. The porous hydrophobic material is packed into a stainless steel chromatographic column, and water is flushed through by means of a high-pressure pump. Back-pressure control is provided by flow restrictors after the column. The applied pressure forces the water to penetrate the pores of the hydrophobic material. The water volume contained in the column, as a function of the applied pressure, reflects the degree of wetting of the solid by water. This volume is determined by measuring, through a mass spectrometer, the retention time of deuterium oxide injected in the column. The method is applied with alkyl-modified (n-octyl and n-octadecyl) and perfluorinated (alkyl and phenyl) silica surfaces. The information gathered by this study may contribute to explain the retention behavior of reversed-phase chromatographic columns when used under highly aqueous mobile-phase conditions.