Promising results in terms of moisture and oxygen permeation barrier properties have been reported for organic/inorganic multilayers, but the impact of the organic interlayer on the overall barrier performance is still under discussion. It is generally accepted that the organic interlayer acts as a smoothening layer, allowing for the decoupling between defects/pinholes present in the polymer substrate and the inorganic layer. It is, however, also hypothesized that the organic interlayer infiltrates into the nano-pores present in the inorganic barrier layer, therefore affecting the barrier properties at microstructural level. In the present work, the moisture permeation barrier performance of SiO2/organosilicon multilayers deposited by means of initiated-and plasma enhanced-chemical vapor deposition is investigated. Calcium test measurements were used to discriminate between the overall water permeation (effective water vapor transmission rate, WVTR) through the layer and the permeation through the matrix porosity (intrinsic WVTR). The improvement in terms of intrinsic barrier performance was found to correlate with the residual nano-porosity content, due to the filling/infiltration of the organosilicon monomer in the SiO2 nano-pores. However, such improvement upon the deposition of the organosilicon interlayer is limited to a factor four. These results, in combination with the analysis of the local defects present in the multilayer structure, lead to the conclusion that the main contribution of the organosilicon interlayer to the overall barrier performance is the decoupling of the above-mentioned local defects/pinholes. (C) 2015 Elsevier B.V. All rights reserved.