It is shown that a fracture test using an asymmetric double cantilever beam test geometry is a powerful tool to study the effect of interfacial reactions on the improvement of the interfacial fracture toughness (G(c)) of immiscible polymer systems. The G(c) values between a partially aromatic polyamide (PA) and a poly(arylether sulfone) (PSU) can be increased significantly when reactive PSUs are used which are obtained by grafting with maleic anhydride by introducing pyromellitic anhydride end-groups or by introducing carboxylic acid groups via copolymerization. Optical and atomic force microscopy investigations of the fracture surfaces show different failure mechanisms for weak and strong. interfaces. For weak interfaces it was possible to determine the crack opening geometry using interference microscopy. For significantly reinforced interfaces rib marking lines on the PSU fracture surface can be observed. X-ray photoelectron spectroscopy (XPS) measurements reveal that with increasing toughness of the interface more and more cohesive failure of the PA occurs. This results in an increasing amount of nitrogen detected an the PSU fracture surface and simultaneously no sulfur is detected on the PA fracture surface.