Ultraviolet photoemission spectroscopy was used to determine the work function phi and the hole injection barriers Delta(h) of interfaces formed between pentacene and two fundamentally different electrode materials: (i) conductive polymers comprising poly(3,4-ethylenedioxythiophene) and sulfonate moieties, and (ii) An with chemisorbed Cl. phi of these substrates before pentacene deposition covers a wide range of 4.25-5.15 eV for the polymers, and extends to 5.8 eV for Au-Cl. Despite this huge variation in phi, Delta(h) after pentacene deposition is found to be almost identical for all cases (approximate to 0.35 eV). The mechanism responsible for Delta(h) being independent of initial surface phi is proposed to be an interfacial charge transfer reaction leading to cationic pentacene species. These findings are compared to interfaces between pentacene and clean metal surfaces, where A h was found to decrease with increasing substrate phi. (c) 2004 Elsevier B.V. All rights reserved.