The kinetics of the amorphous-to-crystalline (fcc) phase transition under isothermal treatments has been investigated in alloys with the composition close to Ge22Sb22Te56, which is the most frequently used for the erasable optical memory devices. For monitoring the transformation, electrical, ellipsometric, optical transmission and electron microscopy measurements were employed. A procedure to determine the transformed volume fraction based on the electrical conductivity data is proposed. It is shown that the widely accepted assumption about the linear dependence between the conductivity variation and the transformed volume fraction is not correct. The use of this assumption could give an error in the determination of the crystalline volume fraction of up to one order of magnitude. The amorphous-to-crystalline transformation kinetics observed agrees with the Johnson-Mehl-Avrami formalism. The transformation exhibits two distinct stages that are attributed to the surface and bulk nucleation, and gives an average Avrami exponent around 2. The activation energy of the process is about 4 eV, which exceeds previous estimations.