In this work we have investigated the dielectric properties of Ag/PTCDA/Ag structures using impedance spectroscopy (IS). An equivalent circuit is developed for the ac small-signal excitation with frequencies between 100 Hz-5 MHz. It is possible to fit the data at different dc biases without introducing any frequency-dependent element to the equivalent circuit. The space charge region at the bottom Ag contact strongly resembles Schottky-barrier behavior, while disorder induced interface dipoles seem to occur at the top Ag/PTCDA interface. For the latter one, the related energy distribution of density of states as deduced from the excess capacitance of this zone fairly matches a Gaussian profile. (C) 2004 Published by Elsevier B.V.