A fluorescence monitoring device was developed to study the distributive mixing process inside the chamber of a batch mixer. A bifurcated UV-fluorescence probe was implemented instead of the thermocouple measuring the temperature of the melt. Hydroxymethyl anthracene was used as fluorescence tracer and was preliminarily dispersed in the minor component. Furthermore, the main polymer of this study was a copolymer of ethylene and vinyl acetate (EVA). Structure development during mixing of miscible polymer blends, low viscosity ratio blends (plasticizer/polymer), and immiscible polymer blends was investigated by this method. It was shown that the mixing process of a miscible system depends on the shear rate, which governs the temperature of the melt and consequently the melting process of the solid masterbatch-tracer at the early stages of mixing. On the other hand, the presence of a miscible component of very low viscosity, as, for example, mixing of DOP in EVA, delays the onset of mixing. This lubricant effect was shown to be due to the fact that DOP migrates to the wall of the chamber rather than inside the polymer, though it is well miscible with the EVA phase. The study of immiscible polymers, as, for example, polypropylene in EVA, did not show any fundamental difference with miscible polymer blends. This observation is actually inherent to the UV-fluorescence tracer dispersed in a masterbatch system, which allows quantification of the distributive mixing process only.