Abstract: Method for forming on a superalloy or other metallic substrate a platinum graded, outward single phase diffusion aluminide coating on a surface of the substrate by depositing a layer comprising Pt on the substrate and then gas phase aluminizing the substrate in a coating chamber having a solid source of aluminum (e.g. aluminum alloy particulates) disposed therein close enough to the surface of the substrate to form at an elevated substrate coating temperature a diffusion aluminide coating having an inner diffusion zone and outer additive single (Ni,Pt)Al phase layer having a concentration of platinum that is relatively higher at an outermost coating region than at an innermost coating region adjacent the diffusion zone.