A theoretical and understanding sf molecular interfaces, such as in organic light-emitting diodes (OLEDs), comparable to that of conventional covalent semiconductor heterojunctions does not exist al the moment. A step in this direction is made here, with the determination via photoemission spectroscopy of HOMO and LUMO levels across a PTCDA/Alq(3) heterojunction. it is shown thar the relative position of the LUMOs is inadequate to confine electrons in the emissive laver (Alq(3)), which explains in part the low luminescence efficiency of PTCDA/Alq(3) OLED devices.