Vibrational and complete high resolution spectra (HREELS) of Langmuir-Blodgett (LB) monolayers of mixtures of pure hydrogenated and deuterated stearic acids on aluminum substrates were recorded. The results show that electrons penetrate into the film and relax in the conduction band of the insulating film. Complete spectra are mainly composed of different regions corresponding to vibrational losses, and to exponential decay of the intensity of backscattered electrons where superimposed structures, fixed to the vacuum level, are observed for primary energies higher than 4 eV. An emission of thermalized electrons is also observed near the vacuum level. Conclusions about the different aspects of this interaction are inferred combining information obtained from the different regions in the spectra.