We show that Al2O3 thin films, grown by atomic layer deposition (ALD) on polyester, are ultrabarriers with moisture permeation <10(-5) g-H2O/m2-day, as determined after aging for more than three years. We present evidence that the mechanism for gas permeation in ALD Al2O3 films is not due to pinholes, but that the onset of permeation occurs abruptly, analogous to electrical breakdown in oxide thin films. We show that the permeation onset time increases for thicker Al2O3 films and higher ALD process temperature, for which the hydrogen defect concentration in Al2O3 films is less. Further, we show that mild plasma treatment of the polyester, prior to ALD deposition of Al2O3, makes the surface more hydrophilic and reduces moisture permeation compared to an untreated surface. Similarly, ALD deposition on the bare or non-slip side of the polyester film is preferred for low permeation.