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We present an investigation of the reinforcement of the interface between a flexible amorphous polymer (polystyrene, PS) and a semicrystalline polymer (a polyamide, Ny6). Poly(styrene-co-maleic anhydride) was used as the compatibilizer. Fracture toughness was measured using the asymmetric double cantilever beam test (ADCB). For bonding temperatures above 190 °C, the adhesion strength was found to increase with bonding time, pass through a peak value, and then reach a plateau. The fracture toughness increased with increasing bonding temperature, passed through a peak near 200 °C, and then decreased with further increase of the bonding temperature. This behavior was more obvious for an amorphous polymer/semicrystalline polymer pair than for a pair of semicrystalline polymers. The variation of the fracture toughness with bonding time and temperature can be plausibly explained in terms of two different failure mechanisms:  adhesive failure at the interface for short bonding times and when the bonding temperature is low, and for longer bonding times and at high temperatures, cohesive failure between chains at the interface and the bulk PS due to decreased chain entanglement. As long as the compatibilizer molecular weight is sufficiently large that entanglements form between the matrix polymers, common features of the fracture mechanisms depending on the bonding conditions can be outlined that are independent of the crystallinity of the polymer.