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The transfer of matrix toughness to composite mode I interlaminar fracture toughness (GIc) has been investigated in unidirectional glass-fibre reinforced composites with brittle and rubber-toughened vinyl ester matrices. Single-edge-notch bend (SENB) and double cantilever beam (DCB) specimens were used for matrix and composite GIc characteristion, respectively. The initial crack opening displacement rate was used as the parameter for comparison of GIc results. Matrix GIc was completely transferred to composite GIc for crack initiation (GIc -init) in the brittle-matrix composites, but in the toughened composites transfer was only partial due to the presence of fibres. The conclusion is that the maximum contribution to energy absorption by the matrix is more accurately reflected by GIc -init, and should be used for further assessment of the enhancing effect of fibre bridging during steady-state crack propagation, instead of matrix GIc. A plot of composite GIc for steady-state crack propagation, GIc -prop versus GIc -init indicates that the enhancing effect of fibre bridging is greater in the toughened composites. This enhancement is related to a larger deformation zone size in the toughened matrices.