A new energy dispersive x-ray diffraction method was used to observe isothermal phase transitions of a polyethylene oxide sample contained in a variable temperature cell. A series of scattering measurements were performed to follow the time evolution of the system at each temperature. The collected spectra were suitably processed and the transition coordinatexwas obtained as a function of time. A detailed treatment of the errors was accomplished to evaluate the reliability of the method. The experimental points (dx/dt)max—Twere plotted both for the amorphous-crystalline (below the melting temperatureTm) and for the crystalline-amorphous (overTm) transitions. They reproduce the usual behavior forT < Tmand show a characteristic unexpected feature forT>Tmwhich is partially justified.