The oxidation life of a physically vapor-deposited overlay protective coating based on the Ni-Cr-Al or Co-Cr-Al (M-Cr-Al) systems is controlled by aluminum consumption resulting from alumina spalling, erosion, and interdiffusion with the substrate. The rates of these processes are determined by coating and substrate composition, service environment, and temperature. The purposes of this study were 1) to determine the effect of coating and substrate chemistry on the kinetics of interdiffusion and their relation to diffusion-zone constitution and 2) to develop a procedure for analyzing diffusion in multicomponent, multiphase systems. Semi-infinite diffusion couples with MCrAl sources representative of coatings and sinks representative of nickel-or cobalt-base gas turbine alloys were annealed at 1000, 1095, 1150, or 1205°C for as long as 500 h. The couples were examined by optical microscopy and layer growth measurements were made. Parabolic growth constants can be correlated with source aluminum content for a specific sink composition since changes in source aluminum content are more important than changes in chromium content. Sink composition is as important as source composition in determining β-recession kinetics in diffusion couples and life in finite coatings. Nickel-base alloys are more stable sources and stronger sinks than cobalt-base alloys. Total and diffusion activation energies were determined from layer growth constants. By introducing the concept of β source strength which can be determined from the appropriate phase diagrams, the Wagner solution for consumption of a second phase in a semiinfinite couple was successfully applied to the analysis of the MCrAl couples. This provided a method 1) for correlating β-recession rate constants with couple composition, 2) for determining reliable total and diffusional activation energies, and 3) for calculating interdiffusion coefficients.