Hybrid (inorganic-organic) polymer materials deposited onto flexible polymeric substrates can provide sufficient barrier response against oxygen and water vapor penetration. In the case that they are combined with inorganic oxide layers (e.g. SiOx, AlOx) in multilayered structures onto flexible polymeric substrates, they can achieve barrier values for oxygen and water vapor below 10(-3) cm(3) m(-2) d(-1) bar(-1) and 10(-3) g m(-2) d(-1), respectively. The in depth knowledge of their optical properties can provide significant insights on their structure and it will contribute towards the optimization of their barrier response. In this work, Spectroscopic Ellipsometry from the infrared to the visible-ultraviolet has been implemented for the investigation of the influence of silicon dioxide nanoparticles, which have been added in the hybrid material during its synthesis, in order to strengthen the inorganic-organic network, on the optical properties of hybrid polymers. The performed analysis revealed valuable information about the electronic and vibrational response as well as the crosslinking mechanisms of these materials. The correlation of the optical properties with the synthesis parameters and the barrier response will contribute towards their optimization in order to be widely used as ultra high barrier materials for flexible organic electronics. (C) 2008 Elsevier B.V. All rights reserved.