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Changes to the Mode I interlaminar fracture toughness, GIc, and fracture mechanisms of stitched and unstitched fibreglass/vinyl ester composites were investigated after exposure to elevated temperatures. The fibreglass was stitched through the thickness with Kevlar®-49 thread in two orientations with two stitch densities, and then resin transfer moulded with a cold-curing vinyl ester resin. After curing at room temperature (∼20°C) for several weeks, the composites were heated to between 100 and 300°C for 1 h or at 175°C for times ranging from 0.25–100 h. The GIc values, which were measured using the double cantilever beam method, of stitched composites in the cold-cured condition were between 1.5 and 2.3 times higher than the unstitched composite. It was observed with scanning electron microscopy that this toughening occurred by deflection of the crack tip at the stitches, by the ability of the stitches to remain intact for a short distance (7–15 mm) behind the crack front, and by partial pull-out of broken stitches. The interlaminar fracture toughness of the unstitched composite increased slightly following heating, despite a possible breakdown of the chemical structure of the vinyl ester between 150 and 300°C. In contrast, the interlaminar toughness of the stitched composites was degraded significantly by heating, and this was probably caused by thermal deterioration of the Kevlar® stitches. This study reveals that the elevated-temperature post-curing of stitched composites will reduce the effectiveness of Kevlar® stitching in raising the Mode I interlaminar fracture toughness.