The influence of mechanical treatment on polymorphism of sulphathiazole and piroxicam has been studied. It was shown by thermal analysis, X-ray diffraction and IR-spectroscopy methods that mechanical treatment leads to the partial amorphization of the drugs and III<->I polymorphic transformations of sulphathiazole. The changes in molecular structure of piroxicam were suggested. The mechanical activation of drugs in mixtures with polyvinylpyrrolidone allowed us to obtain stable specimens of the drugs with elevated rates of dissolution and apparent solubility as compared to intact and mechanically activated substances.