The influence of 6-thioxanthine (6TX) on the thermal transformation of 6-mercaptopurine (6MP) crystals was investigated using scanning electron microscopy and X-ray diffraction. The crystal lattice of 6MP and 6TX molecules can include up to 10% of the counterpart molecules to form a solid solution. Monohydrate crystals (form I) of pure 6MP were directly transformed into anhydrate crystals (form II) by heating at 150-220°C, but similar crystals (form I_TX) of 6MP containing more than 2% 6TX were converted into another form of anhydrate crystals (form III) via an amorphous state by the same treatment. These phase transformations were kinetically studied and there was a clear difference in mechanism under the present experimental conditions. The transformations of form I into form II and form I_TX into an amorphous state were assumed to proceed according to the contracting cube equation whereas that of the amorphous state into form III appeared to follow the Prout-Tompkins equation. The reason for the transformation into different crystal forms, from form I into form II and from form I_TX into form III, is discussed on the basis of the experimental results.