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Distribution maps of each component in the middle stage of granulation (5 min). (a) Mefenamic acid granules; (b) flavoxate hydrochloride granules. Red indicates high concentration and blue indicates low concentration.The present study aimed to clarify, by means of micro-spectroscopy, the mechanism of aggregation of particles into granules during high-shear granulation. We used two types of pharmaceutical granules prepared by high-shear granulator, one containing mefenamic acid and the other containing flavoxate hydrochloride as poorly soluble active pharmaceutical ingredients (APIs). Lactose, cornstarch, and microcrystalline cellulose were used as excipients; and hydroxypropyl cellulose (HPC) was used as the binding agent. The distributions of components in granules were visualized by mapping cross-sections of individual granules with techniques utilizing mid-infrared spectroscopy at the SPring-8 synchrotron radiation facility and micro-Raman spectroscopy. In the distribution maps of mefenamic acid granules, distributions of mefenamic acid, cornstarch, and microcrystalline cellulose overlapped; in flavoxate hydrochloride granules, on the other hand, distributions of flavoxate hydrochloride and lactose overlapped. Assessment of the surface free energy of each component found that ingredients with overlapping distribution had similar surface properties. Therefore, it was revealed that in high-shear granulation, in addition to the granulator operating conditions and general properties of the formulation itself (such as the solubility and particle size of each ingredient), the surface properties of the ingredients and their interrelationships were also factors that determined the aggregation behavior of the particles.