The effects of hydrothermal ageing on the thermomechanical properties of high performance epoxy and its nanocomposite were studied. The epoxy-clay nanocomposite was prepared through a recently developed "slurry-compounding" approach. The cured samples were immersed in distilled water at 60 degrees C for different periods of time before subjecting to characterization. The storage modulus, relaxation behavior, fracture toughness, and tensile properties were investigated. It was found that the storage modulus and a-relaxation were strongly affected by water uptake, while the fracture toughness and Young's modulus were less influenced. Dependence of tensile strength and strain at break on water uptake was found to be different in neat epoxy and epoxy-clay systems.