The relationship between the microstructure and the water vapour transmission rates of aluminium oxide and aluminium coatings deposited by magnetron sputtering on polyethylene terephthalate have been investigated. The gas barrier properties of the films have been measured as a function of temperature and a range of techniques used to characterize the coatings including atomic force microscopy, which also provided information on the early growth mechanism. It was found that the Al/PET film showed a better water vapour barrier than the AlOx/PET although the activation energy for water vapour permeation was the same for both. We propose that the interaction of water with the barrier coating plays a significant part in determining the observed gas barrier performance.