Infrared reflection absorption spectroscopy (IRRAS) was applied to characterize monolayers at the air/water interface and to get information of the molecular structure of the film forming amphiphiles. In the case of lipids, it is possible to characterize the conformation of the hydrocarbon chains and the headgroup structure. Ordered alkyl chains allow the determination of their orientation. The secondary structure of peptides and proteins at the air/water interface as well as the interaction with lipid films can be investigated. In particular cases it is possible to determine the orientation of the respective secondary structures. A program for the simulation of IRRA spectra and for the calculation of the orientation of molecular vibrations was written. Besides the simulation of IRRAS vibrational bands, the whole baseline of the IRRA spectrum could be calculated. Characteristical parameters are the film thickness, the refractive indices and the tilt angle of the molecular director. The OH stretching vibrational band of the water subphase was used for these calculations. The investigation covered the examination of the liquid crystal 8-cyanobiphenyl, several lipids (different deuterated DPPC isomers, sphingolipids [gangliosides and ceramides], bis-(monoacylglycero)phosphates) as well as different lipid/protein systems (lipid A and lactoferrin, POPG and KLAL, GM1 and GM2 activator protein).