The invention relates to a method and device for the determination of the gas permeability of a container, for example, a PET bottle. Said container is enclosed by a sheath, which hermetically seals the container from the environment. The intermediate space, between the container and the sheath, has only a very small volume in comparison to the volume of the container. The determination of the gas permeability is begun by bringing said intermediate space to, for example, atmospheric pressure, whilst the container is filled with a test gas by means of a special feed, until the container is at an overpressure relative to the intermediate space. The pressure in the intermediate space increases by means of the resulting diffusion of the test gas through the wall of the container into said intermediate space. The increase in pressure per unit time is a measure of the permeability of the container.