Solid-state reactions of Mn thin films on GaAs(001) lead to the formation of epitaxially oriented tetragonal Mn2As near the GaAs substrate and a layer of tetragonal delta-MnGa at the surface at annealing temperatures above 300 degrees C. However, an intermediate structure is first formed that has the composition Mn(0.6)Ga(0.2)AS(0.2) with a repeated distance of about 6.3 angstrom perpendicular to the substrate interface. This intermediate structure is metastable relative to the formation of Mn2As and delta-MnGa but provides pathways for allowing Mn to diffuse toward the substrate and Ga to diffuse toward the surface. First principles, density functional theory calculations were used to study the possible structures of this intermediate material. Both vacancy containing and nonvacancy containing structures were considered. Our results indicate that vacancy containing compounds are the most probable. Furthermore, we propose a mechanism for Mn and Ga diffusions and illustrate how interface compound formation occurs in the Mn/GaAs reactions. (c) 2006 American Vacuum Society.