A study has been made of the interlaminar fracture behaviour of unidirectional carbon fibre-epoxy matrix composites containing polyethylene terephthalate (PET) interleaving films, with and without plasma surface treatment. Comparisons are made of the initiation values of mode I and mode II interlaminar fracture toughness, G_Ic and G_IIc, for composites with and without interleaves. It is shown that the interleaves have completely reverse effects on interlaminar fracture toughness with different modes of fracture. The interleaves have the detrimental effect of reducing G_Ic, up to 70%, and the plasma treatment is effective in mitigating the toughness reduction. In sharp contrast, the interleaves improve the corresponding toughness in mode II, with a negligible influence of plasma treatment on G_IIc. The major failure mechanisms associated with the presence of interleaves are discussed.