In this paper the results of a study of the interdiffusion between an LPPS CoNiCrAlY coating with a Ni-based CM186 alloy substrate are shown. Accelerated oxidation testing at 1200degreesC is used to demonstrate the sequence of changes occurring to the microstructure of the interdiffusion zone with increasing time at temperature. The evolving microstructural features have been identified using high resolution scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Elemental profiles of the diffusion zone at different times at temperature are compared to a computational model predicting such profiles. It is found that aluminium ingress into the alloy, as predicted by the model, is inhibited by the formation of Al-rich phases in the interfacial zone. With extended time at temperature, aluminium. depletion within the coating triggers the dissociation of these phases. These then act as reservoirs and supply aluminium back into the coating.