While molybdenum oxide (MoO3) has been shown to be an effective hole transport layer in organic photovoltaic (OPV) devices, a complete understanding of its electronic behavior has proven elusive. In this work, thin films of substoichiometric molybdenum oxide (MoO3–x) were prepared via thermal evaporation and subjected to a variety of annealing conditions. The films were employed as the hole transport layers in organic photovoltaic devices, and the device performance was found to depend strongly on the annealing conditions: as-prepared MoO3–x films produced poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester devices with good performance (3.1% power conversion efficiency), while films annealed at higher temperatures or in a reducing atmosphere produced devices with very low efficiencies (≤1%). Through X-ray absorption near-edge structure (XANES) measurements at the Mo L3-edge, we show that while oxygen vacancies present in the as-prepared films may play a key role in hole extraction, extensiv...