A coating process is described in which a boron-doped nickel layer is first deposited onto a substrate and is then overcoated with aluminum. This duplex coating is annealed such that a nickel aluminide coating is formed on top of a boron-based diffusion barrier. During a low temperature anneal, the boron, which had been evenly distributed throughout the Ni layer, becomes concentrated in a remnant Ni layer which is located between the substrate and a NiAl overlayer. During a high temperature anneal on Ti-alloy substrates, Ti borides are formed. This study reveals two stages in which diffusion is inhibited. First, diffusion is inhibited by the boron-enriched nickel layer. Second, diffusion is arrested by a titanium diboride diffusion barrier. Results of electron and ion spectroscopies are used to illustrate the nature and effectiveness of the diffusion barrier when applied to various Ti-alloy substrates. In this application, it is shown that the diffusion of Ti, V, and Mn, into the NiAl layer is inhibited. Anticipated benefits of this barrier, when employed with oxidation protection coatings, are discussed.