The barrier formation for metal-organic semiconductor interfaces is analyzed within the induced density of interface states (IDIS) model. Using weak chemisorption theory, we calculate the induced density of states in the organic energy gap and show that it is high enough to control the barrier formation. We calculate the charge neutrality levels of several organic molecules: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI) and 4,4',N,N'-dicarbazolyl biphenyl (CBP) and the interface Fermi level for their contact with a An (111) surface. We find an excellent agreement with the experimental evidence and conclude that the barrier formation is due to the charge transfer between the metal and the states induced in the organic energy gap. (C) 2004 Elsevier B.V. All rights reserved.