Despite the benefit of increased operating temperature of turbine blades, the application of thermal barrier coatings (TBC) has brought the concern of coating spallation during jet engine operations. A significant portion of past research, was therefore, devoted to detailed analysis of the interfacial degradation behavior of TBC by numerical and analytical models. Nonetheless, these models tend to overlook the influence of the degradation on the complicated nature of substrate chemical diffusion/phase transformation. As a result, the predicted TBC lifetime expressed in terms of failure criterion (i.e. thermally grown oxide thickness) often deviate largely from that obtained by real experiment. The purpose of this research work is to clarify the role of substrate chemistry on the interfacial degradation of TBC. Extensive analysis was carried out on a set of systematically prepared commercial TBC subjected to different thermal exposure times. The results clearly indicated that the interfacial adhesion is a dynamic property, which depends on the substrate chemical diffusion/phase transformation. It became evident that the early spallation observed in several β bond coat system was caused by two unique phase transformations. Furthermore, the phase transformation was shown to be closely associated with the inherent chemistry of the substrate.