Langmuir monolayers are observed to flow into the interface between hydrophobic organothiol monolayer modified gold surfaces and water, forming a bilayer. From time-dependent capacitance measurements, the rate of bilayer formation (e.g., interface penetration) has been quantitatively determined. The capacitance-time profile can be modeled with a single monolayer flow parameter. The value of the now parameter is sensitive to the Langmuir monolayer, the structure and composition of the hydrophobic surface layer, and the Langmuir film surface pressure. The monolayer now rate is largely controlled by dynamical interactions occurring within the entire bilayer contact region. We find that the lateral flow of the Langmuir film into the hydrophobic surface/water interface is well described by a simple pressure-driven flow model. Possible applications of lateral flow are discussed including the initial design of a monolayer chromatography system.