In situ backside surface enhanced Raman spectroscopy (in situ-SERS) was newly employed for the study of the transport of inhibiting molecules through a polymer film. The barrier properties of layer-by-layer polyelectrolyte films (PE) composed of polyacrylic acid and polyallylamine hydro-chloride layers on Ag-surfaces were compared between untreated, thermally crosslinked, and Ag-nanoparticles containing samples. IB-SERS enabled the study of the transport of 2-mercaptobenzimidazole (MBI) as an inhibitor through the film. Water barrier properties of the treated PE films determined by Electrochemical Impedance Spectroscopy were correlated to the MBI diffusion kinetics. The PE stability against MBI diffusion and thermal treatment was analyzed by Infra-Red Reflection Absorption Spectroscopy (IRRAS). IRRAS showed that the thermally treated PE films formed chemical crosslinking via amide bonds and lowered the diffusion of water and the water uptake in the films. Moreover, the MBI diffusion kinetics can be followed by means of SERS. However, MBI adsorption at the PE film/metal interface was not detected after the heat treatment. In this case the adsorbed PE on the Ag surface was not substituted by the competing adsorption of MBI. Moreover, the presence of Ag-nanoparticles in the film decelerated MBI diffusion to the SERS substrate due to the trapping effect of MBI molecules (C) 2011 Elsevier Inc. All rights reserved.