In this study, the structure of amorphous and crystalline of Ge:Sb:Te:O films, with oxygen content from 0 to about 28 at.%, have been analyzed using X-ray diffraction, impedance measurements and X-ray photoelectron spectroscopy. From these results, the location of the oxygen atoms in the crystallographic structure has been deduced. The results have shown that, in films with oxygen content below 10 at.%, Te, Sb and most of Ge are in metallic state and the free oxygen is probably located at the tetrahedral interstitial sites. In amorphous films with higher contents of oxygen up to 29 at.%, tellurium is fort-ning part of the Ge:Sb:Te alloy, while some of the germanium and antimony form amorphous oxides. This amorphous oxides segregate to the grains boundaries during crystallization. Both, the free oxygen and germanium oxide formed, act as nucleation centers for crystallization. Due to the deficit of germanium and antimony thus created, the amorphous films crystallized as Sb2Te3 in the rhombohedral phase with segregation of the excess crystalline tellurium. Such diffusion-limited process increases the nucleation time in laser-induced crystallization. (c) 2006 Elsevier B.V. All rights reserved.