We investigated vibrational and low-energy electronic excitations in indium/perylene-3,4,9,10-tetracarboxylic dianhydride (In/PTCDA) system, prepared on a cleaved MoS 2 surface, by high-resolution electron energy loss spectroscopy (HREELS). As expected from the previous ultraviolet photoemission studies, HREELS spectra were drastically changed when In was deposited on the PTCDA film. In the vibrational energy loss region, a clear red-shift of C O stretching loss peak of carboxylic group in PTCDA was observed. In the electronic energy loss region, the molecular-exciton energy of 2.6eV in PTCDA was lowered to an unusually low-energy value of 1.0eV by the In deposition. These results originate from the formation of In 4 PTCDA.