The coupling of calorimetric and microscopic techniques shows that the whole crystallization process of a Se85Te15ssy alloy proceeds by two mechanisms which are called surface and bulk. These mechanisms are activated differently depending on the particular heating rate used and on the temperature of the isothermal heat treatment chosen. The nucleation frequency and growth rate are determined from reflection polarized optical microscopy analysis and a good agreement is found between these experimental results and predictions obtained by use of the classical nucleation and the normal growth theories. The relation between the apparent activation energy from the whole crystallization process which is obtained via differential scanning calorimetry and the activation energies of nucleation and growth deduced from microstructural analysis is established. A detailed discussion about the meaning of the different Avram indexes found is presented, and discrepancies between time-temperature-transformation (T-T-T) curves built from both differential scanning calorimetry results and the nucleation frequency and growth rate fits can be explained in terms of these two different mechanisms determined.