The crystallization kinetics of sputtered Ge4Sb1Te5 films has been investigated using temperature-dependent sheet resistance measurements in conjunction with structural investigations employing X-ray diffraction (XRD) and X-ray reflectometry (XRR). This work correlates the kinetics of crystallization of thin Ge4Sb1Te5 films to the structural and density changes. The resistivity of the Ge4Sb1Te5 samples decreases from 2 to 3.1 X 10(-5) Ohm m upon annealing above the phase transition temperature. The transition temperature varies with the heating rate and has been determined to be approximately 453 K. Corroborative XRD patterns attribute the rapid resistivity change to the structural transformation from an amorphous as-deposited state to a crystalline rock salt structure (a=5.974+/-0.002 Angstrom). Furthermore, it has been established that Ge4Sb1Te5 forms no second crystal structure upon annealing to higher temperatures. The crystallization process has been quantified by determining the activation energy, E-u, to be 3.48+/-0.12 eV using Kissinger's analysis. XRR measurements yield density values of 5.59+/-0.02 and 6.14+/-0.02 g cm(-3) for the amorphous and crystalline phases, respectively. This corresponds to a density increase of 9.3%, which causes a contraction of 9.1% in film thickness upon annealing, as revealed by XRR. The formation of an oxide layer has been observed to commence in the amorphous phase at 378 K and saturates for an oxide thickness of 1.7 nm at 413 K.