Rutherford backscattered spectrometry, x-ray diffraction, and cross-sectional transmission electron microscopy were used to examine Mn/GaAs interfacial reactions. Mn films deposited in situ on molecular beam epitaxy (MBE)-grown GaAs(001) epilayers resulted in significant interfacial reactions following post-growth anneals above 200 degrees C. These reactions initially resulted in the formation of a two-phase region of tetragonal Mn2As and tetragonal 6-MnGa, with an average composition of Mn0.6Ga0.2As0.2, and were found to be limited by the rate of Mn diffusion through the reacted region. The two phases formed an epitaxial lamellar layer on the GaAs substrate with Mn2As(001)(100) and delta-MnGa(001)(100)//GaAs(001)(110). Higher temperature anneals resulted in the dissociation of the Mn0.6Ga0.2As0.2 region into a delta-MnGa layer near the sample surface and a Mn2As layer near the GaAs substrate. Results of these reaction studies have been used to make predictions of the Mn-Ga-As ternary phase diagram for temperatures up to 400 degrees C, which indicates that both Mn2As and delta-MnGa are thermodynamically stable in contact with GaAs. (c) 2005 American Vacuum Society.