Ge-Sb-Te films for optical data storage applications were deposited by magnetron sputtering of separate Ge, Sb, and Te targets on Si(1 1 1) wafers in a dc argon plasma. To investigate the influence of the chemical composition of the phase change material on its optical properties, films with lateral compositional gradients of up to 30 at.% were deposited. The composition and structure of the films were investigated by X-ray photoelectron spectroscopy (XPS), electron probe microanalysis (EPMA) and X-ray diffraction (XRD) on plain Si wafers, whereas the phase change velocity of Ge-Sb-Te as a re-writable optical data storage medium was determined on Si/Al/SiO2/Ge-Sb-Te multilayers near to technical conditions. The phase change of Ge-Sb-Te films was induced and characterized with a static tester consisting of an optical microscope with an integrated high power laser diode. The change in reflectivity induced by the laser pulses was measured by a high sensitivity photodetector. Depending on the composition, the crystallization time was determined between 220 and 500 ns, while the amorphization time was between 20 and 120 ns.