The enhancement of interfacial adhesion between immiscible amorphous polyamide and polystyrene (PS) by the addition of thin layers of styrene-maleic anhydride (SMA) random copolymer, used as a reactive compatibilizer, has been investigated. The asymmetric fracture test was employed to measure the fracture toughness of the interface as a function of the SMA amount placed at the interface. The fracture toughness of the interface between amorphous polyamide and bulk SMA remained low at all temperatures and annealing times of our experiment. When a thin layer of SMA was introduced between amorphous polyamide and PS, the fracture toughness was however remarkably increased as the amount of SMA layer and the annealing temperature were increased. This is believed to be due to copolymer formation at the interface due to reaction between the amine end group of the polyamide and the maleic anhydride (MAH) of SMA. A qualitative model is suggested for the combined effect of reaction at the interface and diffusion to obtain a strong interface. On the basis of this model, the experimental results obtained here are successfully divided into two regions : a diffusion-dominant region and a region in which the diffusion and the reaction at the interface are comparable, thus yielding a strong interface.