The interface formation between a metal and MoO3 was examined. We carried out in situ ultraviolet and x-ray photoemission spectroscopy with step-by-step deposition of MoO3 on clean Au and Al substrates. The MoO3 induces huge interface dipoles, which significantly increase the work functions of Au and Al surfaces. This is the main origin of the carrier injection improvement in organic devices. In addition, interface states are observed at the initial stages of MoO3 deposition on both Au and Al. The interface states are very close to the Fermi level, assisting the charge transport from the metal electrode. This explains that thick MoO3 layers provide good charge transport when adopted in organic devices.