Anhydrous theophylline exhibited polymorphism and two modifications, which were named form I and form II, were isolated. The melting point and the enthalpy of fusion of each polymorph determined by differential scanning calorimetry (DSC) were 273.4±1.0°C and 26.4±0.3 kJ/mol for form I and 269.1±0.4°C and 28.2±1.1 kJ/mol for form II. The higher melting form I had a smaller density. In contrast to caffeine, theophylline formed a monohydrate. The dissociation vapor pressure curves of theophylline hydrate and caffeine hydrate were obtained and the difference in their stabilities was discussed. The enthalpy of dehydration was determined by DSC under closed conditions. The dehydration under isothermal conditions appeared to proceed acoording to the mechanism of random uncleation followed by two-dimensional growth of nuclei as represented by the Avrami-Erofe'ev equation. The solubilities of theophylline and its hydrate were determined as a function of temperature. From the van't Hoff type plot, the transition temperature between the hydrate and the anhydrous form was determined.