Chalcogenide alloys are frequently used for rewritable optical data storage where submicron sized regions of the film are switched between an amorphous and a crystalline state. Since the kinetics of this process are crucial for the success of the technology employ a combintion of different techniques to study the transformation. The activation energy for crystallization is determined by measurements of the electrical resistance upon heating. The structural changes upon heating is derived from x-ray diffraction while x-ray reflectometry is employed to measure the density change upon crystallization. For a number of different alloys density and thickness changes upon crystallization between 5 and 10% are observed. These density changes are accompanied by irreversible stress changes which could possibly limit the lifetime of the films. Nevertheless the measured stress change is much smaller than the stress change expected for a purely elastic deformation. This can be explained by a viscous flow of the amorphous pulse which can also account for the changes of film topography upon crystallization is observed by atomic force microscopy. Microscopic measurements of the crystallization kinetics reveal of correlation with the film stoichiometry, in particular the relative abundance of Ge-Te bonds in GeSbTe alloys. Concepts of combinatorial synthesis are employed to identify phase change materials with fast transformation kinetics.