The solid-solid phase transformation of caffeine has been studied with time- and temperature-resolved X-ray powder diffractometry (TXRD) and with emanation thermal analysis (ETA). The phase transformation of caffeine at 141 degrees C has been followed by heating a sample of the low-temperature phase. After cooling again, the high-temperature phase converts only slowly back into the low-temperature phase. This transformation needs weeks or months at room temperature to complete. With TXRD, it was possible to study this slow transformation in situ at elevated temperatures. A nucleation-controlled transformation mechanism appears likely.