The electronic structure of a series of phenyl-capped 3,4-ethylenedioxythiophene oligomers has been studied using ultraviolet photoelectron spectroscopy (UPS) in combination with theoretical calculations. The calculations were performed for isolated oligomers within the framework of density-functional theory, using the Becke+Lee-Yang-Parr exchange-correlation functional. Excellent agreement between the UPS spectra and the quantum-chemical calculations allowed for unambiguous interpretation of the results. We use the asymptotic extension of electronic properties of oligomers to discuss the electronic structure of neutral poly(3,4-ethylenedioxythiophene). In addition, experimentally determined ionization potentials for thin films prepared by vapor deposition and spin coating are reported, and are found to depend on the preparation method. The discrepancy is assigned to differences in the molecular packing. (C) 2003 American Institute of Physics.