We study the phase transition from amorphous to face-centered-cubic (fcc) polycrystalline Ge2Sb2Te5 thin films. The transformation has the peculiar characteristic, still not well understood, of coexisting high transition rates at low annealing temperature (with grain growth velocities of the order of 1nm∕min at 140°C) and large activation energies (about 2–2.5eV). In this work all kinetic parameters governing the transition have been determined by in situ transmission electron microscopy analysis. They are thermally activated and consistently show large anomalies in the preexponential terms of transient and steady-state nucleation rates and grain growth rate. A possible explanation for these observations is described, based on a recently proposed atomic structure of the fcc Ge2Sb2Te5, characterized by a high level of disorder and distortion at the short range.