Polymorphic solid state transition of caffeine anhydrate crystals from a metastable Form I to a stable Form II was conducted to investigate the kinetics as a function of temperature and humidity. The kinetics was expressed well with the penetration model proposed in our previous study. The transition rate was independent of the humidity but was dependent on the temperature. The activation energy was comparable to that for the solid state transition of theophylline anhydrates, indicating a similar mechanism of solid state transition.