Interdiffusion at the interface between a Co-36.5Ni-17.5Cr-8Al-0.5Y, MCrAlY coating and the underlying IN738 superalloy was studied in a large matrix of specimens isothermally heat treated up to 12,000 hours at temperatures 875°C, 925°C or 950°C. Microstructural investigations and calculated phase fraction diagrams show that a precipitate free zone forms between the coating and superalloy and grows with time. The width of the growing zone was estimated on the basis of average intensity profiles obtained from experimental x-ray maps measured using energy dispersive spectroscopy in a scanning electron microscope. A simple parabolic growth model was set up for estimating the metal temperature near the coating/ substrate interface based on the growth kinetics of the precipitate free zone. Parameters for the model were extracted from measurements of the width of the growing precipitate free zone with time. The developed model was used to estimate metal temperatures for a service exposed, first stage gas turbine blade.