The tensile fracture of glass-bead/epoxy composites has been studied using a large number of glass volume fractions (q) from 0 to 0.25. Absorption of water from a saturated NaCl solution was used to destroy interface adhesion. The strength of the dry composites decreased up to q = 0.15, then became constant. The strength of the wet composites showed a plateau around q = 0.12, then an abrupt drop between 0.12 and 0.15, then became nearly constant. It is possible that the abrupt drop in strength occurs at the percolation threshold. However, electrical measurements on similar composites prepared with conducting (silver-coated) beads showed that the applicable percolation threshold (qc) was ≥ 0. 16, in agreement with the observation of anomalous water uptake only for q ≥ 0.18. It is concluded that the abrupt drop in strength occurs below qc, and is due to large finite interface clusters (or rather the more planar parts of such clusters) acting as nuclei for a progressive matrix fracture process. The plateau around q = 0.12 is due to favourable fibre-like interactions of smaller interface clusters, and gives the highest normalized (wet/dry) strength of about 0.9.