The water and fuel absorption and desorption characteristics have been studied of aircraft fuel tank sealants. The sealants were an epoxy-cured polythioether and a MnO2-cured polysulphide. The effect of the water and fuel exposure on the glass transition temperature was also investigated. Water diffusion was found to be Fickian for the polythioether sealant and the equilibrium plateau level was found not to be dependent on the immersion temperature. On the other hand, the polysulphide sealant demonstrated a clear non-Fickian water uptake characteristic, with large uptake levels, which were attributed to water droplet formation in the volume of the bulk sealant. The glass transition temperature of the polythioether sealant was not affected by the presence of water (similar to 4% by weight) while that of the polysulphide was increased by similar to 8 degrees C for a water uptake level of similar to 50% by weight. Fuel absorption followed a very similar trend for both sealants with the weight gain demonstrated an early maximum followed by a subsequent weight decrease, indicating leaching of fuel soluble fractions, until equilibrium was reached. The presence of fuel acted as plasticizer for the sealants; depressing their glass transition temperature by similar to 10 degrees C. Tests on dried samples have shown that the effect on the glass transition temperature was reversible. (C) 2008 Wiley Periodicals, Inc.