The water absorption characteristics and the degradation of mechanical properties of a unidirectional glass/epoxide composite have been studied during hygrothermal ageing at 95-100% relative humidity and temperatures. up to 80[degrees]C. Initial water absorption behaviour approximately followed Fick' s prediction at temperatures up to 45[degrees]C. At 80[degrees]C substantial deviation from Fick's laws occurred from the beginning. No equilibrium water uptake level could be obtained at any of the temperatures studied. Using Scanning Electron Microscopy, unreacted curing agent was detected as heterogeneities embedded in the resin matrix. Also, various microstructural defects, in the form of voids, cracks and debonding, were observed, and these changes are explained by postulating a degradation mechanism involving interaction between the unreacted curing agent and water. The absorbed water lowered the 3-point flexural strength and 'the short beam horizontal interlaminar shear strength significantly at all temperatures studied. The fall in 3-point flexural modulus of specimens exposed at temperatures up to 45° C was negligible, but specimens which were exposed at 80[degrees]C showed a moderate fall. 4-point bending stresses were applied to some specimens during ageing. These stresses did not affect the diffusivity, but Induced damage on the compressive surfaces. The extent of damage degraded the mechanical integrity further. Pre-stressing had no effect, but post-loading wet specimens caused fibre kinking. Removal of absorbed water restored the fibre-resin interface and much of the loss in flexural properties. However, surface damage caused by applied loads increased the loss in residual flexural strength and modulus. A supplimentary short project carried out with an unidirectional carbon/epoxide composite Showed much simpler absorption charateristics at 80[degrees]C and 95-100% R.H. The retention of the flexural modulus. of wet specimens tested at 80[degrees]C was extremely good, but the loss in flexural strength was substantial.