Ni-base single crystal (SC) superalloys containing high concentrations of refractory elements are prone to generate a diffusion layer called Secondary Reaction Zone (SRZ) beneath their bond coating during exposure at high temperatures. SRZ causes a reduction of the load bearing cross section and is detrimental to the creep properties of thin-wall turbine airfoils. In this study, a new coating system – “EQ coating”, which is in thermodynamic equilibrium with the substrate, has been proposed and the formation behavior of SRZ beneath bond coat materials was investigated on the 5th generation Ni-base SC superalloy developed by NIMS. Diffusion couples of several alloys were made and were heat treated at 1100°C for 300 h, 1000 h. The concentration profiles were analyzed by EPMA. Also, cyclic oxidation tests were carried out at 1100°C in air.