Two-dimensional laminated composite materials are susceptible to damage by interlaminar crack growth. Delaminations severely degrade composite load-carrying capability. Interleaved composites are fabricated with thin films inserted between composite prepreg plies. Previous work has shown that interleaved composites have significantly higher delamination and impact resistance than composites without interleaves. This paper examines interleaving as a technique for the improvement of delamination resistance in composites. In particular, the influence of the deformation behavior of the interleaf film and film adhesion on composite fracture toughness is studied. Teflon and Kapton interleaf film materials are compared. Kapton displays predominantly brittle deformation behavior. Teflon exhibits substantial deformation before failure. Both films are plasma treated to vary film-resin bond strengths. The fracture toughness of composites interleaved with these films is determined. Plasma treatments are shown to increase film-resin adhesion and fracture toughness. The effectiveness of Teflon as an interleaf material may be limited by the toughness of this film. Fracture toughness specimens are found to fail at the film-plasma grafted polymer interface. Additional optimization of the film-grafted polymer bond is necessary for improved performance.