The physicochemical properties of drug molecules in a mixture with montmorillonite or pillar interlayer montmorillonite (PILM) were studied by using differential scanning calorimetry (DSC), powder X-ray diffraction and infrared (IR) spectroscopy. The DSC and IR data indicated that benzoic acid was converted to ionic species by heating to 400K in PILM200 and PILM400 mixtures, whereas benzoic acid molecules partially existed in an amorphous state in both montmorillonite and PILM600 mixtures.The decomposition rate of aspirin in the montmorillonite mixtures was studied at 40°C and 0, 31.3 and 79% relative humidities (RH). Asprin was decomposed rapidly at 0% RH as compared with 79% RH in the PILM mixtures. This result indicates that the aspirin did not react with the bulk water but with active water adsorbed on the internal pore surfaces of PILM.