This study aims to develop a concept of reactive compatibilizer-tracer, which allows one to study the evolution of the morphology and interfacial reaction of reactive polymer blends in a large-scale twin-screw extruder (TSE). A reactive compatibilizer-tracer is a compatibilizer precursor whose polymer chain bears reactive groups capable of reacting with at least one of the polymer components of a polymer blend. It also bears moieties capable of being analytically detected with great accuracy. This concept is shown for polystyrene (PS)/polyamide 6 (PA6) blends using a copolymer of styrene (St), 3-isopropenyl-alpha,alpha'-dimethylbenzene isocyanate (TMI), and 9-(methylaminomethyl)anthracene (MAMA) as a reactive compatibilizer-tracer (PS-TMI-MAMA). While its isocyanate groups are capable of reacting with the terminal group of the PA6 forming a graft copolymer of PS-g-PA6-MAMA, its anthracene moieties allow for measuring very small amounts of the reacted reactive compatibilizer-tracer or in situ formed graft copolymer by gel permeation chromatography (GPC) with an ultraviolet (UV) detector. The use of the reactive compatibilizer-tracer together with transit experiments provides a tool for studying reactive blending processes in a twin-screw extruder.