There are currently no techniques that allow <i>facile</i> on-line determination between amorphous and crystalline material. Herein, a new colorimetric method, perichromism, has been developed to distinguish between amorphous and crystalline material, based on diffuse reflection ultraviolet-visible (UV-visible) spectroscopy. A probe molecule, phenol red, at 0.1% w/w concentration was added to sucrose and dissolved in water. The solution was freeze-dried, which resulted in an amorphous excipient with the probe molecule incorporated in an intimate mixture. The samples were then stored at a selected controlled temperature and relative humidity to allow control of crystallization of the sucrose. The amorphous or crystalline nature of each sample, both with and without the addition of the phenol red probe molecule, was monitored through X-ray diffraction, differential scanning calorimetry, and Raman spectroscopy. It was found that sucrose remained amorphous when stored in an atmosphere with a relative humidity below 32%; above this level, crystallization occurred. It was found that the presence of the probe molecule did not alter the extent of crystallinity of sucrose. Analysis of the samples containing the phenol red probe by diffuse reflection spectroscopy revealed that spectra could be classified into two groups depending on the λ<sub>max</sub>. Furthermore, correlation of the diffuse reflection spectra with X-ray diffraction and differential scanning calorimetry data showed that sucrose was amorphous in one group and crystalline in the other group, thereby revealing that diffuse reflection spectroscopy can, when phenol red is used as a probe molecule, quickly and easily determine whether sucrose is crystalline or amorphous.

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