The stress corrosion cracking of glass fibre-reinforced plastic (GFRP) accompanies a phenomenon of catastrophic failure as a result of the rapid fall in strength owing to corrosion breaking of the glass fibres. This produces a flat surface without pulling fibres out of the plane. Attack on the glass fibres can only occur by contact with an acid which must first diffuse into the matrix resin. It is confirmed, however, that no diffusion occurs or that it is too slow to be detected. The relationship between fatigue damage and stress corrosion in an acidic environment, has been investigated, focusing on the effect of matrix toughness on the resistance to stress corrosion failure of GFRP. Three types of GFRP, made from matrices with different toughness, were studied after subjecting them to fatigue damage at different levels.