Understanding the role of interactions between proteins and surfactants provides insight into the mechanism of molecular recognition and the role of binding cooperativity in the protein structure. Fibrinogen is interesting because it is a major inhibitor of lung surfactant function at the lining layer of the alveoli. Recently we have characterized the complexation of fibrinogen with three different surfactants: sodium octanoate, dodecanoate and perfluorooctanoate in the bulk and at the air-water interface. The differences in alkyl chain length, stiffness and hydrophobicity promote a variety of electrostatic, hydrophobic and steric hindrance forces which results in different stability and structural film patterns. To understand these systems better we now require a direct characerization of the composition and structure of the adsorbed layer in situ at the air-water interface. The logical next step in this project is therefore a set of neutron reflectivity measurements using isotopic contrast variation which here we propose to carry out on FIGARO.