Intermetallic interlayers are key to the performance of thermal barrier coating systems. The role of platinum group metal (PGM) additions to B2 bond coat interlayers has been investigated with emphasis on diffusional aspects of coating structure evolution and properties. Additions of Ru and Ir to NiAl reduce interdiffusion and the resultant thickness of the coating and interdiffusion zone. Relative to Pt and Pd, Ru and Ir slow diffusion. Ru additions to NiAl increase creep resistance but degrade oxidation kinetics.