Model epoxy resin formulations were modified with different concentrations of liquid rubber and preformed crosslinked rubber particles and impregnated into unidirectional carbon fibers. The liquid rubber was used to increase the mode I fracture toughness of the interlayer toughened prepreg systems, which already have high mode II fracture properties. In this paper it is shown that the mode II fracture toughness is not sacrificed by the incorporation of carboxyl terminated butadiene acrylonitrile rubber (CTBN) in the matrices, while mode I fracture toughness can be increased by as much as 100%.