We present an experimental study of polymer-polymer reaction kinetics at the interfaces between two immiscible polymer phases under flow in a batch mixer of type Haake Rheocord. To that end, we have developed a model chemical system that is composed of a mixture of polystyrene (PS) and poly(methyl methacrylate) (PMMA). A small fraction of PS bear hydroxyl terminal group (PS-OH) and that of PMMA contain nonclassical isocyanate moieties that are randomly distributed along the PMMA chains (PMMA-r-NCO). This reactive system is particularly pertinent to modeling practical reactive blending processes because the amount and rate of copolymer formation can be determined with great accuracy ton the order of ppm). This study shows that the overall reaction rate is controlled primarily by interfacial generation through convective mixing. Most reaction and morphological development are accomplished within a very short period of time (1-3 min). For a PS/PMMA (60/40) reactive blend, the ultimate size of the PMMA particles is as small as 0.2 mu m and is reached within 2 to 3 min. A surface coverage of about 0.5 of the PMMA particles by a monolayer of the copolymer is enough to prevent dynamic coalescence, whereas a much higher surface coverage is needed to eliminate static coalescence. In the nonentangled regime (M-n of the PS-OH = 7800 g/mol, temperature has a significant effect on the reaction rate, while it has little effect in the entangled regime (M-n of the PS-OH = 53,200 g/mol).