We investigated vibrational and low-energy electronic excitations in indium/perylene-3,4,9,10-tetracarboxylic dianhydride (In/PTCDA) system, prepared on a cleaved MoS2 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 PTCDAfilm. 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 2.6 eV in PTCDA was lowered to an unusually low-energy value of 1.0 eV by the In deposition. These results originate from the formation of In(4)PTCDA. (C) 2003 Published by Elsevier Science B.V.