We experimentally investigate the isothermal amorphous-to-fcc polycrystalline phase transition process in amorphous Ge {sub 2} Sb {sub 2} Te {sub 5} thin films prepared by sputtering. The amorphous layers were either as deposited or formed by Ar {sup+} ion implantation after crystallization at 300 deg. C. The kinetics of the amorphous-to-polycrystal transition are analyzed through electrical measurements, in which the Johnson-Mehl-Avrami-Kolmogorov theory is employed. The procedure to extract the kinetics of the phase transition from conductivity versus time data is carefully discussed and compared to data of cross-sectional transmission electron microscopy images versus anneal time. By following this proposed procedure, the nucleation and growth parameters, and the activation energies have been determined. Results indicate that the process of isothermal crystallization in Ge {sub 2} Sb {sub 2} Te {sub 5} takes place in two stages, in which the Avrami exponent changes in the range from 3 to 1. These results are understood in terms of modifications in the kinetics of the phase transition.