Structural composite materials based on epoxy resin and carbon fibers reinforcement are widely used in high performance applications because of their low density and high mechanical properties compared to metallic structures. However, the highly cross-linked network of such matrices induces poor fracture toughness and low impact damage resistance. Literature suggests incorporation of additives mainly toughener agents such as thermoplastics (TP), block copolymers or elastomers to enhance the performances of the final composites. Additives are mostly incorporated in a specific thermosetting resin using different concentrations and mechanical characterization tests are performed on a particular lay-up… Comparison of the results is not always straightforward because specimens are produced in different ways. The overall objective of this work is to compare a high performance epoxy based reference panel and composite panels modified with various tougheners (Phenoxy, Polyethersulfone (PES), MAM block copolymers and CTBN rubber) which are made in the same conditions in terms of thermosetting resin, concentration, lay-up, process, etc. The relationship between the microstructures resulting from reaction induced phase separation and the delamination toughness properties of the investigated model compounds (without carbon fibers) are first evaluated. Then, mechanical properties such as energy release rate (GIC), compression after impact (CAI), interlaminar shear strength (ILSS), etc. are performed on neat and modified prepregs based composite panels and results are compared. Keywords: Epoxy resin, Toughening agents, Polymer blends, Mechanical testing.