The formation of thin films and monolayers of (3-aminopropyl)triethoxysilane (APS) on aluminum and gold substrates was studied with reflection-absorption FTIR spectroscopy (RAIR), ellipsometry, contact-angle, and quartz-crystal microbalance (QCM) measurements. The structure of the APS films is strongly dependent on the experimental conditions, such as substrate pretreatment, mode of adsorption, and posttreatment. APS was adsorbed on the substrates from aqueous solution, from refluxing solvent, and from the gas phase. Adsorption from solution yields multilayers. APS films of about molecular thickness are obtained by vapor adsorption on the precleaned substrate. The presence of water has a strong effect on the film formation. Depending on the experimental conditions, the final surface loading varies between about 5.3 and 8 molecules per nm(2), and the film thickness varies between 5 and 11 Angstrom.