A process for forming a diffusion aluminide coating on an article, such as a component for a gas turbine engine. The process is a vapor phase process that generally entails placing the article in a coating chamber containing an aluminum donor material, without any halide carrier or inert filler present. The aluminum donor material consists essentially of about 20 to about 70 weight percent aluminum, with the balance being chromium or cobalt. While the article is held out of contact with the donor material, coating is initiated in an inert or reducing atmosphere by heating the article and the donor material to vaporize the aluminum constituent of the donor material, which then condenses on the surface of the article and diffuses into the surface to form a diffusion aluminide coating on the article.