Poly(ethylene terephthalate) substrates were coated with thin films of silicon oxide deposited by magnetically enhanced chemical vapor deposition. The rates of oxygen and water vapor transport through the coated and uncoated film systems were obtained as a function of temperature. Activated rate theory treatment of oxygen transmission rates revealed that the silicon oxide coatings were imperfect; the apparent free energies of activation (Delta E(p)) for transport through film substrates which were coated on a single side were statistically identical to uncoated controls. However, coating both sides of the polymer substrate with identical oxide layers resulted in a 54 kJ/mol increase in the Delta E(p) value. A simple empirical model for the change in transport mechanism is offered to explain this unanticipated result. Analogous treatment of water vapor transport rates for these same film systems showed no obvious change in transport mechanism. However, Delta E(p) values obtained for water vapor permeation through silicon oxide-coated poly(ethylene terephthalate), polystyrene, polypropylene, and polycarbonate substrates were identical within experimental error, suggesting attractive interaction between the oxide layer(s) and water.