This paper presents the results of an investigation of the effects of hygrothermal conditioning on the mechanical properties and fatigue behaviour of epoxy-based composites reinforced with carbon, glass and aromatic polyamide fibres. Cross-plied (0$^\circ$/90$^\circ$) laminates of these materials, of nominal fibre volume fraction ca. 0.6, were conditioned by drying, by exposing to a 65% r.h. (relative humidity) atmosphere and by boiling in water. The effects of these treatments on the tensile and shear strengths and on the tensile fatigue response of the laminates when tested orthogonally and at $\pm$ 45$^\circ$ to the fibre lay-up are discussed. The 0$^\circ$/90$^\circ$ tensile properties of the three laminates are relatively little affected by the conditioning treatments except for g.r.p. (glass reinforced plastic) exposed to boiling water, and of K.f.r.p. (Kevlar fibre reinforced plastic) in which the strength is reduced somewhat by complete drying. The shear strengths and $\pm$45$^\circ$ tensile strengths are more sensitive to the effects of moisture, however, and it appears that optimum strengths are developed after conditioning at the intermediate 65% r.h. Tensile fatigue tests have been carried out on laminates in the 0$^\circ$/90$^\circ$ and $\pm$45$^\circ$ orientations. In c.f.r.p. (carbon fibre reinforced plastic) there is no effect of conditioning on the fatigue behaviour of 0$^\circ$/90$^\circ$ samples, and in g.r.p. only the boiling water treatment affects the results. Preloading g.r.p. samples before conditioning has no effect on their fatigue behaviour. The tensile fatigue resistance of 0$^\circ$/90$^\circ$ Kevlar-epoxy laminates is reduced by drying more than by boiling, and in all conditions the stress against logarithm of life curves are characterized by a downward curvature that renders the long term stability of this material suspect. This behaviour is also reflected in differences in the $\pm$45$^\circ$ tensile fatigue response of the three laminates.