Silicon oxide (SiOx) coatings have been exploited for packaging applications due to their exceptional properties. They can be fabricated by a number of techniques, including Electron Beam Evaporation (EBE); an easily applicable method in industrial scale. The SiOx films' thickness and stoichiometry are the key parameters for the achievement of the final functional properties of the systems. The need to reduce material waste as well as the time required for the evaluation of the systems, make real-time monitoring and control requisite. This work highlights a methodology based on real-time Multi-Wavelength Ellipsometry for the monitoring of the EBE processes of SiOx coatings on poly(ethylene terephthalate) membranes. It is presented a thorough correlation between the basic optical parameters, such as refractive index and Penn gap, and the SiOx films' thickness, stoichiometry. composition and functional properties, such as oxygen permeation. (C) 2009 Elsevier B.V. All rights reserved.