It is very challenging to investigate a reactive compatibilization process because both mixing and interfacial reactions between reactive polymers are highly coupled. Most importantly, the amount of the in-situ formed copolymer is often too low to be determined in a relatively accurate manner. For this reason, we propose a concept called reactive compatibilizer-tracer. It bears reactive groups capable of reacting with its counterpart upon forming a copolymer in-situ, and fluorescent labels allowing determining very small amounts of the in-situ formed compatibilizer. This paper reports on an unusual and highly undesirable phenomenon which may occur during a reactive polymer blending process. When a critical amount of the in-situ formed compatibilizer is reached, the dispersed phase domains undergo a drastic increase in size with a further slight increase in the amount of the compatibilizer, instead of continuing to decrease or leveling off as one may expect. This phenomenon is revealed owing to the concept of reactive compatibilizer-tracer. It indicates that a reactive compatibilizer can be a very efficient compatibilizer under certain thermo-mechanical conditions and may become inefficient under other ones. It also shows how challenging it may be to design and optimize the molecular architecture of a reactive compatibilizer as a function of thermo-mechanical conditions under which a reactive blending system is processed. The reactive polymer blending system used to reveal the above phenomenon is composed of polystyrene (PS) and polyamide 6 (PA6) which bears a terminal amine group. The reactive compatibilizer-tracer is a polystyrene which bears along its chain and in a random manner a few weight percents of isocyanate and a very small amount of 9-(methylaminomethyl)anthracene. There are three advantages for choosing this specific reactive compatibilizer-tracer, the most important one being that a few ppm of anthracene can be determined in an accurate manner.