The electronic structures of vacuum-deposited molybdenum trioxide $\left({\text{MoO}}_3\right)$ and of a typical ${\text{MoO}}_3$ /hole transport material (HTM) interface are determined via ultraviolet and inverse photoelectron spectroscopy. Electron affinity and ionization energy of ${\text{MoO}}_3$ are found to be 6.7 and 9.68 eV, more than 4 eV larger than generally assumed, leading to a revised interpretation of the role of ${\text{MoO}}_3$ in hole injection in organic devices. The ${\text{MoO}}_3$ films are strongly $n$ -type. The electronic structure of the oxide/HTM interface shows that hole injection proceeds via electron extraction from the HTM highest occupied molecular orbital through the low-lying conduction band of ${\text{MoO}}_3$ .