Complimentary spectroscopic, microscopic and electrochemical studies were performed to characterise the barrier properties as well as the interface structure of model iron substrates covered with thin plasma polymer films. Cathodic plasma polymers were deposited which show high barrier properties. The metal surface was pre-treated by a reducing or oxidising plasma. This allowed the adjustment of the oxidation state of the interface layer. The interface structure was characterised by means of X-ray photoelectron sputter profiles, infrared spectroscopy and the application of a Kelvin probe. The investigations show that the measured Voltapotential on the plasma polymer surface can be correlated with the oxidation state of the interface. Reducing plasmas lead to an almost oxide free surface. After deposition of the plasma polymer, this reduced state of the oxide is sensitive to re-oxidation of the interface by oxygen that diffuses through the plasma polymer. It could be shown that the increase in the plasma polymer thickness leads to an improved barrier effect as indicated by the more negative Voltapotential. The barrier properties of the organosilicon plasma polymer layers on iron were further quantified by means of electrochemical impedance spectroscopy (EIS). The morphology of the plasma polymer surface was studied using atomic force microscopy (AFM) to support the evaluation of the impedance data. A water diffusion coefficient of 4 x 10(-14) cm(2)/s was found for the plasma polymer. Moreover, the water uptake of the film was measured to be 0.13%. These values illustrate the high barrier properties of these thin films. (C) 2004 Elsevier B.V. All rights reserved.