Compression of mixed Langmuir monolayers made from combinations of dipalmitoyl phosphatidylethanolamine (DPPE) and the semi-fluorinated alkane C8F17C16H33 (F8H16) results in expulsion of the diblock molecule at high pressure. Depending on the DPPE/F8H16 molar ratio, either a monolayer or a bilayer of F8H16 is formed on top of a DPPE-only monolayer. The structure of the hydrocarbon and fluorocarbon layers that constitute the resulting vertically separated bi- or trilayer was determined using grazing incidence X-ray diffraction. The phenomenon is reversible and represents a novel case of vertical, pressure-induced phase separation.