Electrical doping is perceived as the key to enhance the performance and versatility of organic molecular devices.@footnote 1@ Yet, few systematic investigations of the electronic structure of molecular films and interfaces doped with organic molecules have been published to date. We report here an investigation of controlled doping of zinc phthalocyanine (ZnPc) co-evaporated on Au with a strong acceptor, tetrafluoro-tetracyano-quinodimethane (F@sub 4@-TCNQ), using ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES). The 5.2 eV ionization energy of ZnPc is smaller than the electron affinity of F@sub 4@-TCNQ, suggesting host HOMO-to-guest LUMO charge transfer. Undoped ZnPc exhibits near mid-gap Fermi level (E@sub F@) and flat bands away from the Au interface, indicative of quasi-intrinsic purity. In ZnPc doped with ~3% (molar ratio) F@sub 4@-TCNQ, E@sub F@ shifts toward the HOMO level by 0.72eV and reaches ~ 0.16 eV above the leading edge of ZnPc HOMO, as measured from the surface of a 100Å film. At the interface with Au, the ZnPc HOMO is 0.72 eV below E@sub F@, leading to a depletion region with a 0.56eV band bending away from the interface, consistent with the p-type character of the film. The width of the depletion region in doped ZnPc is measured at approximately 32Å, consistent with a simple electrostatic model based on the doping concentration and a dielectric constant @epsilon@=3. The interface dipole barrier between Au and doped ZnPc is of the same sign and similar magnitude as for the undoped material. No evidence of chemical interaction can been seen, suggesting that pure charge transfer is the more likely mechanism for doping. The narrow depletion region in the doped layer is likely to lead to an increase in the tunneling of holes through the junction. I-V measurements will be performed to confirm this point. @FootnoteText@ * Work supported by the NSF (DMR-0097133) @footnote 1@Zhou et al., Appl. Phys. Lett., 78, 410 (2001).