Temperature-modulated differential scanning calorimetry reveals distinct differences in the kinetics of the low-temperature phase transitions of polytetrafluoroethylene. The triclinic to trigonal transition at 292 K is partially reversible as long it is not complete. As soon as the total sample is converted, supercooling is required to nucleate the reversal of the helical untwisting involved in the transition. The trigonal phase can be annealed in the early stages after transformation with a relaxation time of about 5 minutes. The dependence of the reversing heat capacity on the modulation amplitude, after a metastable equilibrium has been reached, is explained by a nonlinear, time-independent increase of the heat-flow rate, perhaps caused by an increased true heat capacity. The order-disorder-transition at 303 K from the trigonal to a hexagonal condis phase is completely reversible and time-independent, It extends to temperatures as low as the transition at 292 K or even lower. Qualitatively, the thermal history and crystallization conditions of polytetrafluoroethylene do not affect the transition kinetics, that is, melt-crystallized film and as-polymerized powders show similar transition behaviors, despite largely different crystallinities. (C) 2001 John Wiley & Sons, Inc.