Transparent barrier coatings on polymers are receiving much attention in industry, for pharmaceutical, food and beverage packaging applications. Plasma-enhanced chemical vapor deposition (PECVD) is among several competing techniques which can produce thin layers of inorganic glassy barrier materials. In this article we describe the performance of silicon compounds (SiO2 and Si3N4) on 13 mu m polyethylene terephthalate (PET) substrates, the barrier coatings being deposited in a dual-frequency (microwave/radio frequency) pilot-scale PECVD reactor for continuously moving flexible webs up to 30 cm in width. The volatile silicon compound used for SiO2 deposition is HMDSO (C6H18Si20O), while SiH4 serves to deposit Si3N4 Coating thickriesses, d, in the,range 8 nm less than or equal to d less than or equal to 200 nm, are measured using a variety of techniques, namely stylus profilometry, continuous, wavelength optical interferometry, x-ray fluorescence, variable angle spectroscopic ellipsometry, and transmission electron microscopy, while film compositions are determined by x-ray photoelectron spectroscopy. Oxygen transmission (OTR) and water vapor transmission (WVTR) measurements are carried out with MOCON ''Oxtran'' and "Permatran-W'' instruments, respectively. As also reported by other workers, we typically find OTR values of about 0.5 scc/m(2) day and WVTR about 0.3 g/m(2) day, for barrier thicknesses exceeding a "critical'' value (d(c), about 15 nm), but the minimum permeation values depend-upon the concentration of defect sites in the coating (mostly related to substrate microroughness). In order to confirm this correlation, we have developed a technique combining reactive ion etching through the PET, followed by optical and transmission electron microscopies, to characterize the types and number densities of coating defects. On the basis of these, we find good agreement between measured and calculated values of OTR.