We have developed a novel in situ Fourier transform infrared (FTIR) spectroscopic imaging technique that can probe the changes in the chemical structures around the matrix-filler interface of polymer composites during reactive processing. We have applied this imaging technique to maleic anhydride grafted styrene-ethylene butylene-styrene triblock copolymer (MA-SEBS) melt-mixed with silica spheres modified with aminopropyl groups (AmSS). In situ FTIR spectroscopic imaging during the heating process indicates that the C=O component, attributed to the maleamic acid group, is generated by a reaction between maleic acid and amino groups at temperatures exceeding 175 degrees C around the matrix-filler interface. It thus appears that the formation of interfacial covalent bonds between matrix and filler contributes to mobility restriction in MA-SEBS molecules around the interface, resulting in the improved tensile strength observed in MA-SEBS/AmSS. In-situ FTIR spectroscopic imaging of this type can provide a better understanding of the reinforcement mechanism of composites by reactive processing.