A toughener composition comprising: a) a polyol component selected from the group consisting of a polyether polyol, a polyester polyol, a polycaprolactone polyol, a hydroxyl-terminated polybutadiene, and mixtures or copolymers thereof; and b) a core shell rubber comprising a rubber particle core and a shell layer wherein said core shell rubber has a particle size of from 0.01 μm to 0.5 μm is disclosed. The toughener can be used in epoxy resin compositions for composite applications.