The objective of this study was to develop a simple, direct and non-destructive method to assess crystal purity of mebendazole raw material and to establish its stability in a suspension formulation using diffuse reflectance ultraviolet (DRA-UV) spectroscopy and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Quantitation of mebendazole, found to exhibit polymorphism with three polymorphic forms A, B and C identified, was carried out with ATR-FTIR spectroscopy. Artificial neural network (ANN) was employed as a data-modelling tool. The developed ANN models confirmed that the characteristic absorptions in the infrared (IR) spectral region are directly proportional to the measured amounts of mebendazole crystal forms present in the samples (r(2)>0.94), which was confirmed with X-ray diffraction (XRD) at r(2)>0.97. These models also predicted that the mebendazole raw material contained 7.21+/-1.25% (ATR-FTIR data) and 10.38+/-0.18% (XRD data) of form A as an impurity. ATR-FTIR data for the suspension formulation showed some dissolution of form C and recrystalisation as the more stable form A. These quantitative results obtained for the binary crystal form mixtures clearly demonstrate the strong potential of ATR-FTIR for use in the determination of the polymorphic content not only in bulk pharmaceuticals but also in liquid formulations.