Because of the advantageous synergistic effects of two alloying elements such as Cr and Al, or Cr and Si, in minimizing the oxidation and corrosion of steels at high temperatures, it would be beneficial to codeposit two elements into diffusion coatings at the alloy surface. But the large differences in the thermodynamic stabilities of the volatile halide molecules for these elements generally preclude their dual introduction at the desired concentrations. However, computer-assisted thermodynamic calculations of the complex equilibrium of vapour species in a cementation pack permit the identification of combinations of masteralloys and halide salt activators which may effect codeposition. For a variety of ferritic and austenitic steels, codeposition of Cr plus Al, or Cr plus Si, has led to surface compositions with excellent oxidation resistance, and in some cases excellent sulfidation and aqueous corrosion resistance, and even hot erosion resistance. The principles for codeposition are discussed, along with some specific examples of particularly effective coatings on steels.