Molybdenum trioxide (MoO3) has been widely used as an interlayer between electrode and functional organic layer inducing significant improvement in the performance of organic electronic and optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). In this work, the electronic structures and energy level alignment at FePc/MoO3 (3 nm)/ITO and FePc/MoOx (3 nm)/ITO (x < 3) interfaces have been comparatively investigated in situ by means of photoemission spectroscopy (PES) in an attempt to understand the influence of MoOx layer on the interfacial hole injection property. It is found that the electron injection barrier is only 0.45 eV for electron extraction from the highest occupied molecular orbital (HOMO) of FePc to the conduction band of MoOx. Simultaneously, two gap states at binding energy of 1.15 eV and 2.30 eV present in 3 nm MoOx, which extend from the valence band maximum of MoOx to the Fermi level. These gap states are suggested to originate from the partial occupation of Mo 4d states due to the reduction of MoO3 during the annealing process. Both lowered electron extraction barrier and presence of interfacial gap states appear to assist the hole injection from ITO anode into FePc by the insertion of MoOx layer. (C) 2013 Elsevier B.V. All rights reserved.