The aim of this work is to study the mechanism of the amorphous-to-crystalline phase transformation in Ge1Sb2Te4 films doped with oxygen using nanosecond laser pulses under isothermal annealing in the time scale of minutes. Experimental results show that the nucleation time, t(nuct) (minimum laser pulse duration for starting the laser-induced crystallization) depends on the oxygen concentration in the films. For those films with compositions in the range of 2-8 at.%, t(nucl) is shorter than that observed in films free of oxygen. In contrast, in films with oxygen in the range of 10-28 at.%, t(nucl) is longer than in the reference sample. Reflection and X-ray measurements on minute time scale have shown that in the films without oxygen annealed under isothermal conditions, the nucleation of the Ge1Sb4Te7 metastable phase is first observed, which is subsequently transformed into the Ge1Sb2Te4 crystalline phase. This effect increases the nucleation time in laser-crystallized materials. For films with 2-8 at.% of oxygen the first nucleation phase to be observed is the crystalline Ge1Sb2Te4, where it is assumed that the oxygen acts as the center of nucleation, therefore decreasing the nucleation time. In the films with oxygen concentration above 10 at.%, the thermal treatments leads to the formation of stable amorphous GeO2. This decreases the amount of available Ge and leads to the formation of crystalline Sb2Te3. The phase segregation in films with more than 10 at.% of oxygen, results in an increase in the nucleation and crystallization times in laser-induced crystallization. This crystallization behavior allows the possibility of having multilevel laser recording. (c) 2005 Elsevier B.V. All rights reserved.