A novel method of measuring the permeation of oxygen through thin films based on the oxidation of aluminium is described and results obtained for polycrystalline thin films of tetracene are reported. Thus for a tetracene film 375 nm thick the permeation was determined to be 0.39 × 10 14 molecules sec -1 cm -2, a rate sufficiently large to cause the formation of additional interfaces in multilayer devices incorporating reactive materials, thereby inducing a drift in device characteristics. The results suggest that permeation is associated with the high density of grain boundaries.