The structural, mechanical and gas barrier properties of aluminium oxide and indium tin oxide coatings deposited by DC reactive sputtering on poly(ethylene terephthalate) has been investigated. The oxygen and water vapour transmission rates of the films were measured as a function of temperature in order to assess the effectiveness of the oxides as gas barriers. Both types of composites exhibited good resistance to water vapour transmission. However, the indium tin oxide coating was significantly more effective as a gas barrier than the conventional aluminium oxide. The microstructural integrity of the oxide coatings was examined using scanning electron microscopy and atomic force microscopy. It was found that the sputtered layers featured a relatively low density of defects. The aluminium oxide films featured larger and many more defects than the indium tin oxide composites. This difference, along with a preliminary observation of a difference in the chemical interaction of water vapour with indium tin oxide (ITO) and aluminium oxide (AIO(chi)), is believed to offer an explanation to the observed gas barrier properties. In addition to displaying good gas barrier characteristics, the ITO films were found to have better resistance to mechanical damage than either aluminium oxide and silicon oxide films.