Interdiffusion during growth of Fe on As-rich GaAs(001) substrates has been investigated by real time stress measurements. Compared to Ga-rich GaAs(001), interdiffusion processes are decisively reduced. The optimum growth temperature (characterized by abrupt interfaces, pseudomorphic growth and negligible intermixing) is found to lie below 50 degrees C. At higher growth temperatures interdiffusion effects increase and eventually lead to the formation of a compact crystalline alloy layer of presumably Fe(2+x)Ga(1-x), as evidenced by transmission electron microscopy and x-ray diffraction.