Thin films of 3,4,9,10-penylenetetracarboxylic dianhydride (PTCDA) were used as an interlayer for the electronic modification of Ag/n-GaAs(100) Schottky contacts. The electronic properties were investigated recording in situ current-voltage (I-V) and capacitance-voltage (C-V) characteristics. For H-plasma treated substrates the effective barrier height decreases from 0.81 to 0.64 eVas a function of the PTCDA layer thickness (d(PTCDA)). In the case of the sulphur passivated GaAs the effective barrier height first increases and then decreases, the overall range being 0.54-0.73 eV. The substrate treatment leads to a different alignment between the band edges of the GaAs and the molecular orbitals of the PTCDA, making it possible to determine the energy position of the LUMO transport level. (C) 2002 Elsevier Science B.V. All rights reserved.