The electronic structures of tris(8-hydroxyquinoline) aluminum (Alq(3)) deposited on clean or Cs pre-covered Ag substrates have been studied by ultraviolet photoelectron spectroscopy. Interface of Cs deposited on Alq(3) has also been prepared for comparison. For a low coverage of Cs on Ag, deposition of Alq(3) on top of the Cs cannot induce any new electronic features. The low work function of the Cs reduces the barrier height of electron injection at the Alq(3)/Cs/Ag contact to 0.3 eV, as compared to 1.6 eV for the Alq(3)/Ag contact: For high Cs,cove. rage, the Cs may diffuse as neutral atoms and undergo oxidation into the Alq(3) layer and form a new gap state at 0.9 eV above the Alq(3) highest occupied state, which is the same as that of Cs deposited on the Alq(3). (C) 2002 Published by Elsevier Science B.V.