The kinetics of moisture absorption have been studied by immersing poly-( butylene terephthalate) (PBT) and its short glass fibre reinforced composites (SGF-PBT) in water at 30, 60, and 90°C. A single free phase model of absorption was applied to the kinetics data and the diffusion coefficient or diffusivity D was evaluated. The value of D was found to be strongly dependent on the water immersion temperatures and to a limited extent on the volume fraction of fibres. The incorporation of fibres resulted in a reduction of the equilibrium moisture content M m . The nature of the interaction of water molecules with PBT and SGF-PBT was investigated by immersing both materials in water at 90°C for a prolonged period. Variation of the pH of the water with immersion time, FTIR spectra taken on the water extract, and SEM examination of the specimen surface, revealed the occurrence of hydrolysis.