We present a High-Resolution Electron Energy Loss study of an 11-mercaptoundecanoic acid self-assembled monolayer (SAM) adsorbed on Au(111). This work has to be placed in the general context of the study of thin organic layers by HREELS and of low-energy interaction mechanisms of electrons with large organic molecules. Evidence is presented here for two resonance phenomena, at similar to1.5 eV and at similar to6 eV, respectively. The first one is attributed to a negative ion resonance involving the carboxylic group, and the second one is the well-known resonance of CH2. From the angular distribution of the elastically scattered electrons the crystalline domain size of the monolayer is estimated to be about 100 Angstrom.