Using a variety of spectroscopic techniques, a number of molecular functionalities have been studied in relation to the adsorption process of proteins to air-water interfaces. While ellipsometry and drop tensiometry are used to derive information on adsorbed amount and exerted surface pressure, external reflection circular dichroism, infrared, and fluorescence spectroscopy provide, next to insight in layer thickness and surface layer concentration, molecular details like structural (un)folding, local mobility, and degree of protonation of carboxylates. It is shown that the exposed hydrophobicity of the protein or chemical reactivity of solvent-exposed groups may accelerate adsorption, while increased electrostatic repulsion slows down the process. Also aggregate formation enhances the fast development of a surface pressure. A more bulky appearance of proteins lowers the collision intensity in the surface layer, and thereby the surface pressure, while it is shown to be difficult to affect protein interactions within the surface layer on basis of electrostatic interactions. This work illustrates that the adsorption properties of a protein are a combination of molecular details, rather than determined by a single one. © 2004 Wiley Periodicals, Inc.