The mechanism of the fast reversible change between the amorphous and crystalline phases in (GeTe)1−x(Sb2Te3)x (GST) has not yet been fully understood. The crystalline phase has been identified as having a NaCl-type cubic structure with random occupation of the A sites by Ge, Sb and vacancies, and 100% occupation of B sites by Te. This fact calls our attention to a possible close relation to the inherent crystal bonding instability observed for the average five valence electrons (<5>) family. We present here the results of systematic hard X-ray photoemission experiments on GST films with various compositions in both the amorphous and crystalline phases, and discuss that a similar chemical bonding instability does indeed play an essential role in the phase change mechanism in GST. We propose a model for the fast phase change, in which 6 fold to 3 fold transition of p-like bonding play an essential role, in this class of materials.