The phase transformation in chalcogenide-based nonvolatile memories is studied by cell electrical characterization. The cell state (amorphous, crystalline, or mixed) is changed by applying electrical pulses, then the cell resistance R and the current-voltage characteristics are measured. From the analysis of the electrical parameters of the cell, we provide evidence for a stacked-like phase distribution in the active layer. Results are discussed with reference to the thermal profile during the program pulse in the chalcogenide layer.