Many commercial polymer blends are manufactured by reactive blending processes. Most of the time, developing a new polymer blend is amount to developing a new compatibilizer. However, the compatibilizing efficiency of a compatibilizer depends not only on its molecular structure and its concentration but also on thermo-mechanical conditions to which it is subjected to during processing. It may happen that a reactive compatibilizer showing good compatibilizing efficiency for a laboratory scale extruder may turn out to be inefficient for an industrial scale extruder. However it is not always convenient to evaluate the compatibilizing efficiency of a new compatibilizer in an industrial process as it may require a very large amount of the compatibilizer. The concept of reactive compatibilizer-tracer is developed to meet the above challenge and can be summarized as follows. Fluorescent moieties such as anthracene are attached to the reactive compatibilizer molecules so that the latter can be served both as a compatibilizer and a tracer. When evaluating its compatibilizing efficiency for a polymer blending system, unlike the polymer compoments of the blend which are continuously fed to the extruder the reactive compatibilizer-tracer is added as a pulse. The concentration of the reactive compatibilizer in the polymer blend at the die exit is measured, in-line and in real time, using probes capable of detecting the signal of the emission of fluorescent moieties of the reactive compatibilizer-tracer. In the meantime, the corresponding size of the dispersed phase domains of the blend is determined off-line. These two pieces of information allow assessing the compatibilizing efficiency of a reactive compatibilizer in a much easier manner and using a much smaller amount of compatibilizer. Consequently the concept of reactive compatibilizer-tracer can help select most appropriate compatibilizers under real industrial polymer blending conditions as well as optimizing them.