Abstract: Fibre hybrid composites are gaining more and more application. So the effects of moisture on hybrid composites are widely concerned. The moisture absorption characteristics is important because it is the basis of investigating the moisture effects. Hybrid composites have several structure parameters that the single composites haven't, such as hybrid ratio (HR). Hybrid interface number (HIN) and sequence of layup(SL). The diffusion of water in hybrid composites must be affected by these parameter. Single (glass, carbon and kevlar) and hybrid (glass/carbon, carbon/fcevlar with different HR, HIN and SL) unidirectional composites with two types of matrixes (epoxy and BMI) are fabricated. The weight change of these materials with time are tested under 80°C water immersion condition. The diffusion coefficient (Dx) and equilibrium moisture con-tent(Mm) are obtained from the tests. It is found that the water diffusion in single and hybrid composites satisfies the Fickian process. The Dx and Mm values of hybrid composites are between two single composites' and increase with the increase of the content of the single composite which has a higher Dx and Mm values. The increasing of the HIN of hybrid composites decreases the Dx but doesn't affect the Mm obviously. The sequence of lay-up doesn't affect either Dx or Mm.in addition, the step-by-step model and thermal model are developed to estimate the Dx and Mm of hybrid composites. The estimsted values are in good agreement with the experimental ones.

Key words: composites, hybrid, absorption, environmental effect, diffusion