The phase transition of anhydrous caffeine form I --> form II was investigated using isothermal microcalorimetry (IMC) and the kinetic results were compared with X-ray diffraction (XRD) studies. As the Arrhenius relationship was assumed to be valid in this study, the comparison was made by defining typical concentration vs. time curves and by generating Arrhenius plots based on times for different degrees of the transformation. The reverse transition was investigated with a differential scanning calorimeter (DSC) to estimate the value of the transition enthalpy Delta H-trs of the polymorphic transformation in question. The selected kinetic equations fitted in IMC data gave great variation in the values of Delta H-trs though the curves correlated usually quite well with the measured data points. Changes in the transition mechanism as the transition progresses were verified from IMC data via the technique, where the calorimetric heat flow signal dq/dt is plotted as a function of released energy Q for different measuring temperatures. The activation energies at specific degrees of the transformation were calculated from the plots of In(dq/dt) vs. 1/T at corresponding energies. The values for activation energies increased as the transition progressed indicating changes in the transition mechanism. The tendency was obvious according to the XRD measurements, too. However, the kinetics obtained with IMC and XRD differed from each other remarkably, which is due to the differences in the detection principle and provides further information on the transition mechanism itself. The nonlinear correlation between IMC and XRD quantitative results showed the transformation to begin at regions with high stored energy.