The aim of this article is to study the mechanism of the amorphous-to-crystalline phase transformation in Ge1Sb2Te4 alloys doped by oxygen (in the range of 4–28 at. %) using the electrical, optical, and x-ray measurements. Experimental results have shown that samples with oxygen in the range of about 4–10 at. % first crystallize into the Ge1Sb2Te4 fcc phase at temperatures in the range 130–145 °C, at higher temperature (around 275 °C) the fcc phase is transformed into the Ge1Sb2Te4 hexagonal phase. In samples with oxygen in the range of 10–15 at. % the crystallization occurs in two stages producing samples where Ge1Sb2Te4 and the Sb2Te3 coexist. Samples with higher oxygen content, in the range of 15–28 at. % the films crystallized into the rhombohedral Sb2Te3 phase due to the formation of amorphous germanium oxide. The experimental results have been interpreted employing the Johnson–Avrami–Mehl–Kolmogorov theory. Using isothermal measurements and the time dependence of the Avrami exponent, three different crystallization mechanisms are proposed for material with different content of oxygen. © 2004 American Institute of Physics.