High resolution vibrational spectra of Langmuir films of octadecyltrimethoxysilane (C18TMS) have been obtained by Inelastic Electron Tunneling Spectroscopy (IETS) in order to identify chemical modifications of C18TMS occurring at the air-water interface when the pH of the subphase is changed. The Langmuir films have been transferred onto an aluminum electrode by the Langmuir-Schaefer technique at three different acidic pH : 1.3, 3.3, and 3.8. The former was expected to promote the formation of polysiloxane polymers, while the latter was presumed to considerably slow down the condensation reaction, although the hydrolysis is effective. These assumptions found a direct proof in the vibrational fingerprint of C18TMS. Finally, we show that IETS is the most appropriate technique for the vibrational study of such films.