The effect of reaction rate on the morphology of reactive blends has been studied using 75/25 (wt/wt) monocarboxylated polystyrene [PS-mCOOH]/poly(methyl methacrylate) [PMMA] with poly(methyl methacrylate-ran-glycidyl methacrylate) [PMMA-GMA] as an in-situ compatibilizer, by varying the amount of PMMA-GMA in the blend, the molar concentration of GMA, C-GMA,C-0 in PMMA-GMA at fixed molecular weight, and the molecular weight of PMMA-GMA at fixed C-GMA,C-0. For the blends with PMMA-GMA having lower C-GMA,C-0, there exists a critical amount of PMMA-GMA above which a sharp decrease in the surface area average domain size (D-s) occurs. This amount was shifted to a smaller value with increasing C-GMA,C-0 in PMMA-GMA. We demonstrated that the interfacial graft reaction between PS-mCOOH and PMMA-GMA at 220 degreesC was described by the simple second-order reaction kinetics, i.e., mean field reaction kinetics. From the morphological evolution, it is found that the morphological change by an external flow from a pellet size to D-s with less than 1 mum occurred within a very short time of similar to 30 s. After this transition, coalescence is the main mechanism for determining the final morphology obtained at a mixing time of 20 min. Finally, a master curve is obtained when D-s is plotted against C-GMA,C-0, implying again that the concept of mean field reaction kinetics adequately applies to the blends employed in this study.