The authors investigate the role of reduced molybdenum trioxide [MoO3-x (x <= 1)] films in organic light-emitting diodes, particularly from the viewpoint of the oxidation state of Mo. MoO3-x films were deposited by reactive sputtering under a mixture of argon (Ar) and oxygen (O-2). The O-2 gas-flow ratio (GFR) [O-2/(Ar+O-2)] was adjusted between 10% and 100%. Mo with six, five, and four valence electrons was detected in MoO3-x film deposited with an O-2 GFR of 10% and 12.5%, whereas, under higher O-2 GFRs, only six valence electrons for Mo in the MoO3-x film were detected. N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (alpha-NPD) layer, hole-transport material, were deposited over the MoO3-x layer by subsequent vacuum evaporation. At the alpha-NPD/MoO3-x interface, it was found that alpha-NPD cations were generated and that MoO3-x was reduced, which provided evidence of charge transfer across the interface by Raman spectroscopy and x-ray photoelectron spectroscopy. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3328822]