Crystallization properties of the phase change materials GeSb xTe and Ge xSb 2Te were studied. For GeSb xTe we found that for large x = 6 the crystallization was very fast but crystallization temperature was low while for small x = 1 crystallization was slow but crystallization temperature was high. An alloy with a good compromise between crystallization time and temperature can be found at x = 4.6, with still very fast crystallization (about 50 ns) and high crystallization temperature of 200 °C. Time resolved X-ray ray diffraction shows that these alloys crystallize in a single rhombohedral phase similar to Sb. For Ge xSb 2Te we found that alloys richer in Ge have higher crystallization temperatures. Ge-rich alloys also have higher resistances in the amorphous phase and a larger electrical contrast. They crystallize in the hexagonal Sb 2Te phase. For both materials system alloys with the highest Ge content show the appearance of the Ge(111) diffraction peak indicative of elemental segregation. © 2011 IEEE.