The structural properties and interfacial organization of polyamides adsorbed as thin films were investigated. Polarization-modulation infrared reflection-absorption spectroscopy was used first to identify the crystalline structure of adsorbed layers and to reveal the conformation and orientation of adsorbed chains. The influence on the structure and molecular orientation of the number of carbon atoms in the aliphatic chain of the diacid part was investigated (PA 66, 610, and 612). The effect of the substrate surface chemistry was also examined. Gold substrates are either inert or functionalized. with OH groups. Grafting of the OH functions did not affect the orientation of chains that apparently lay rather parallel to the interface, whereas the crystalline morphology was dependent on the substrate chemical functionality. Knowledge of the structure and orientation of chains adsorbed at an interface was of fundamental importance for the prediction of adhesive strength, that is, the final performance of these latter depended strongly on the properties of the interface formed between the solids brought into contact.