High-resolution electron-energy-loss spectroscopy (HREELS) and ultraviolet photoemission (UPS) measurements of the interface of phenyl diamine (NPB) and indium tin oxide (ITO) were performed. The HREELS spectrum of the oxygen-plasma-treated ITO was characterized by dipolar-active-phonon modes at 71 and 134meV. Upon deposition of NPB on ITO, the HREELS loss peaks of ITO gradually diminished, while the NPB-derived loss peaks appeared. The negligible energy shifts of the NPB peaks with NPB thickness suggested a rather weak interaction between NPB and ITO. The remarkable intensity change of the loss band at 71-65meV gave evidence for a strong vibrational coupling between the ITO phonon mode at 71meV and the NPB-derived vibrational mode at 65meV. Annealing of the NPB/ITO interface with 20