Poly(ethylene oxide) (PEO) and poly(perfluorohexylethyl methacrylate) (PFMA) containing amphiphilic block copolymers have been investigated for their interfacial behavior and their penetration into 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) monolayer at the air/water interface by measuring surface pressure (pi)/area (A) isotherms in conjunction with infrared reflection absorption spectroscopy (IRRAS). The pi/A isotherms of the block copolymers show pressure regimes corresponding to different conformations of the polymer chains, i.e., a pancakelike conformation at low surface coverage and a brush conformation at high surface pressures. The plateau in pi/A isotherms of the block copolymers, (i.e. a phase transition from the pancakelike conformation to the brush conformation) becomes less and less pronounced with a decrease in the PEO block length. The pi/A isotherm of the DPhPC monolayer shows only a liquid expanded phase state at the air/water interface at all surface pressures. The DPhPC monolayer penetration by the block copolymer chains was investigated by measuring WA isotherms and IRRA spectra of the pure DPhPC, and the block copolymer penetrated DPhPC monolayers. The behavior of the block copolymer chains at the water surface in the presence of the DPhPC monolayer during compression was monitored by following the IRRA signals [(-log(R/R-0), where R is the reflectivity of the sample and R-0 is the reflectivity of the pure water surface] of the v(O-H) and the v(C-O) vibrational bands of the water subphase and the PEO chains, respectively. The intensity of the v(O-H) band increased with compression of the fully expanded film, and the negative v(C-O) band increased also in its absolute value. However, after reaching a maximum value at a surface pressure of approximately 26 mN/m, the intensity of both bands decreased again with further compression. The initial intensity increase in the reflection-absorption bands with compression was attributed to the increased surface density and the subsequent stretching of the PEO chains in the water subphase, forming a more dense and extended conformation. The subsequent decrease is due to the expulsion of the block copolymer chains from the lipid monolayer. At pi approximate to 33 mN/m (where the v(C-O) reflection-absorption band from PEO was not detected any more) all the polymer chains are probably squeezed out of the lipid film.