An investigation has been made of the structure and properties of synthetic polypeptide monolayers spread at the air/water interface. Two series of high molecular weight polymers have been examined, esters of polyglutamic acid and polymers with hydrocarbon side-chains. The structure has been investigated by measurement of the force/area relations and surface potentials with a Langmuir trough, and by measurement of the exchange rates of peptide deuterium. These direct methods have been supplemented by observations by infrared spectroscopy and electron diffraction on collapsed films removed from the surface. It is found that in all cases the properties of the monolayer are consistent with a structure consisting of condensed ordered arrays of $\alpha$-helices. The collapse of a monolayer to form a bilayer causes either a plateau or an inflexion in the force/area curve. From the pressure required to form the bilayer and the work of adhesion between the polymer and water, obtained from Young's equation, the free energy of the polymer/vapour interface has been calculated. The deuterium exchange measurements show that exchange can take place in an intact $\alpha$-helix, and how the exchange rate is influenced by the accessibility of the peptide group to water, the pH of the substrate and the hydrophobic nature of the side chain; the conclusions are important in the interpretation of deuterium exchange in proteins.