The interaction of caesium with spin-coated poly(p-phenylene vinylene) (PPV) layers on Au-coated Si substrates has been investigated. X-ray photoelectron spectroscopy results reveal diffusion of Cs into the polymer layer. Changes in the C(1s) and O(1s) core-level spectra indicate that the alkoxy side-chains of PPV are likely involved in the reactions at the interface. The changes in electronic properties like ionisation energy and work function are followed, using ultraviolet photoelectron spectroscopy and contact potential difference measurements. Only minor changes are found in the ionisation energy upon caesiation whereas the work function decreases by 1.7 eV. Thus, the Fermi level moves from mid-gap to above the lowest unoccupied molecular orbital, indicating n-type doping of the top-most surface region. The barrier for electron injection disappears suggesting that electrons are injected very efficiently from the Cs electrode into the PPV. At the same time, Cs induces gap states in the forbidden band gap that might give rise to non-radiative recombination and thus limit the overall device efficiency. (C) 2004 Elsevier B.V. All rights reserved.