The electronic polarization energies, P = P+ + P-, of a perylenetetracarboxylic acid dianhydride (PTCDA) cation and anion in a crystalline thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both experiments and theory show that P is 500 meV larger in a PTCDA monolayer than in 50 Angstrom films. Electronic; polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within molecules. (C) 2002 Elsevier Science B.V. All rights reserved.