The temperature-induced phase transition in an as-deposited amorphous Ge2Sb2Te5(GST) thin film was studied by a unique combination ofin situsynchrotron techniques (diffraction and reflectivity of X-rays) and sheet resistance measurements. The combination of these characterization techniques allowed the simultaneous extraction of structural (density and film thickness) and electrical characteristics of the GST film during its thermal annealing. It is shown that, at 425 (3) K, the appearance of diffraction peaks associated with a metastable crystalline cubic phase is unambiguously correlated to a density increase in combination with a layer thickness reduction and a resistivity switch towards a lower-resistance state. Under the present annealing conditions, the Ge2Sb2Te5film consists of a polycrystalline layer capped by an amorphous layer that strongly degrades the electrical conductivity.