In this paper the temperature dependence of reversible and irreversible polarization contributions in the temperature range between 223 K and 473 K is investigated to elucidate the microscopic mechanism responsible for reversible and irreversible changes of the ferroelectric polarization. Small signal capacitance measurements under dc-bias are used to measure the reversible contributions. Quasi-static hysteresis measurements are used to obtain the total polarization, i.e. the sum of the reversible and irreversible contributions. The combination of both measurements allows the separation of the reversible and irreversible parts. The reversible and irreversible contributions are demonstrated for SrBi2Ta2O9 (SBT), SrBi2Ta2-xNbxO9 (SBTN) thin films and compared to tetragonal PbZr0.3Ti0.7O3 (PZT) thin film capacitors.