Polytetrafluoroethylene can be considered as a model for calorimetric studies of complex systems with thermodynamics transitions at ambient temperature. This polymer exhibits two phase transitions of different nature at 292 Kand 303 K. We show that sensitive ac-calorimetry measurements allow us to study the thermodynamic behaviour of polytetrafluoroethylene when it is brought out of thermodynamic equilibrium. Thanks to the thermal modelization of our calorimetric device, the frequency-dependent complex heat capacity of this polymer is extracted. The temperature and frequency variations of the real and imaginary parts of the complex beat capacity are obtained when polytetrafluoroethylene undergoes its first-order structural phase transition at 292 K. (C) 2007 Elsevier B.V. All rights reserved.