Reactive compatibilizing blending, called also as in-situ compatibilization, involving interfacial reactions leading to in-situ formation of graft or block copolymers as compatibizers is a convenient and effective way to produce new materials with combinations of properties not available in a single polymer. Twin screw extruders (TSE) are often used for carrying out polymer blending processes. It is very challenging to investigate a reactive blending process in TSE because both mixing and interfacial reactions are highly coupled. Most importantly, the amount of the in-situ formed copolymer is often so low that it is very difficult to determine it in a relatively accurate manner. In this work, we propose a new concept called reactive compatibilizer-tracer, which bears reactive groups capable of reacting with its counterpart upon forming a copolymer for in-situ compatibilization of a reactive polymer blend and at the same time fluorescent labels allowing determining very small amounts of the in-situ formed compatibilizer. To show its usefulness and potential applications, blends based on polystyrene (PS) and polyamide 6 (PA6) are used. The reactive compatibilizer-tracer is a random copolymer of styrene (St), 3-isopropenyl-α, α’-dimethylbenzene isocyanate (TMI) and 9-(methylamino-methyl)anthracene (MAMA), denoted as PS-TMI-MAMA. Using the reactive compatibilizer-tracer together with transit experiments, the influences of various parameters including screw configuration, feeding mode, blend composition and screw speed on the evolution of morphology and interfacial reaction. Those results provide many fundamental insights into reactive blending processes in TSE.