Open spaces in polyethylene terephthalate (PET) coated with aluminum silicate (AlxSi1 (-) O-x(y)) were probed using monoenergetic positron beams. The energy distributions of the annihilation. rays and the positron lifetime spectra were measured for 60-nm-thick AlxSi1 (-) O-x(y) (x = 0-1) deposited on PET using electron-beam evaporation. A clear correlation was obtained between the line-shape S parameter of the annihilation. rays, water vapor, and oxygen transmission rates. The results suggest that open spaces in the AlxSi1 (-) O-x(y) layers play an important role in water/oxygen diffusion in AlxSi1 (-) O-x(y)/PET, where the concentration/size of such spaces showed its minimum value at x = 0.2-0.3. The free volume fraction below the interface between AlxSi1 (-) O-x(y) and PET (region width = 100-400 nm) was decreased by the deposition of the coating layer, and this region was also considered to act as a barrier against penetrations of gasses and molecules. (C) 2013 Elsevier B.V. All rights reserved.