Growth of epitaxial ferromagnetic metal contacts on compound semiconductors with atomically abrupt interfaces is often a challenge due to thermodynamic instabilities at the interface between the two materials and the subsequent formation of solid state reaction products. Elemental ferromagnetic metals, such as Fe and Co, can be grown by molecular-beam epitaxy (MBE) as single crystal films on GaAs; however, they are not thermodynamically stable and reacted phases form at the interface. The initial nucleation of Fe on GaAs surfaces is strongly influenced by the GaAs surface reconstruction, but results in little disruption of the reconstruction itself. Fe/GaAs reactions are reduced at lower MBE growth temperatures with a reacted layer thickness of approximately three monolayers at 15°C. Post-growth anneals at 250°C do not result in the reaction of additional GaAs, but the anneals significantly alter the electronic properties of the interface. Co is more reactive than Fe on GaAs and forms a reaction region composed of Co@sub 2@GaAs, CoGa, and CoAs. Thermodynamically stable metals such as ErAs can be used as epitaxial diffusion barriers to minimize Fe-Ga-As and Co-Ga-As interfacial reactions for growth temperatures as high as 225°C. This paper will emphasize the correlation between the structure, chemistry, magnetism and transport properties of Fe@sub x@Co@sub 1-x@/GaAs and Fe@sub x@Co@sub 1-x@/ErAs/GaAs contacts as determined by STM, RHEED, LEED, XPS, RBS, XRD and TEM. Supported by: ONR, DARPA, NSF/DMR, and AFOSR.