Highly efficient gas-diffusion barriers based on nanolaminates of alternating Al2O3 and ZrO2 layers grown at 80 degrees C by atomic-layer deposition are presented. Ultralow water-vapor permeation rates are reported, and a dramatic reduction of statistical defects on larger areas was found compared to single Al2O3 layers. This study provides a concept for the encapsulation of organic optoelectronic devices.