The effect of reactive compatibilization on adhesion at the interface between the immiscible polymers polystyrene and poly(2-vinylpyridine) was investigated. Reactivity was incorporated into the system by lightly sulfonating the polystyrene component, providing acid functionality which is coreactive with the basic pyridine nitrogen. The interaction between sulfonic acid and pyridine groups generates interchain cross-links and therefore compatibilizing graft copolymers. The reinforcing effects of in-situ copolymer generation on the interface were quantified by evaluating interfacial adhesion through measurement of interfacial fracture toughness using the asymmetric double cantilever beam test. Interfacial properties were assessed as a function of the concentration of sulfonic acid groups present. The results show that the toughness of the interface is dramatically improved by the incorporation of reactivity, increasing from similar to 2 J/m(2) in the unreactive system up to >150 J/m(2) in the reactive system. At long reaction times, the toughness of the reactive interface approaches that of pure polystyrene. Approximately 5-7% sulfonation was found to produce optimum mechanical reinforcement. Scaling relations between the amount of compatibilizer formed at the interface and interfacial fracture toughness suggest that the copolymers formed act to initiate interfacial crazing during failure, accounting for the enhanced adhesion.