The influence of fibre surface treatment on the mode I delamination characteristics of a glass fabric/vinyl ester composite was studied. Five treatments were used: solutions containing 0.01wt%, 0.4wt% and 1.0wt% of γ-methacryloxy-propyltrimethoxysilane (MS); 0.4wt% of MS subsequently washed in methanol; and 0.4wt% of γ-glycidoxypropyltrimeth-oxysilane (ES). Static mode I tests were performed on specimens oriented in both the warp and weft fibre directions. The tests revealed that stable crack propagation was only observed with the ES-treated specimens and with the most dilute MS treatment. Invariably, fracture toughnesses were higher in the weft oriented specimens. The degree of unstable fracture observed under fatigue loading was generally significantly lower than under static loading. The specimens treated with the highest concentrations of MS possessed the highest fatigue resistance and had threshold toughness values in excess of the static toughness values. Fractographic examination was performed on both static and fatigue fracture surfaces, revealing markedly different fracture morphologies in relation to the degree of interfacial failure observed.