The transition temperature and the heat of transition between the hydrate and the anhdrous forms of phenobarbital were determined to be 36.4° and 1.87kcal/mole, respectively by the initial dissolution rate measurements. These values were in good agreement with values obtained by the conventional solubility equilibrium method and by the differential scanning calorimetry (DSC), as well as with transition temperature estimated by the electrical conductivity and vapour pressure measurements. Furthermore, metastable solubility values calculated from experimentally determined stable solubility values and ratios of dissolution rates for both forms agree closely with values obtained by the conventional solubility method. The assumption that the dissolution processes for both forms of phenobarbital are diffusion controlled was further supported in this study by the equality of dissolution activation energy for both forms, as well as by the magnitude of this activation energy, and by the diffusion layer thickness.