Ultrathin Fe films were epitaxially grown on Ga- and As-terminated GaAs(1 0 0) at room temperature by molecular beam epitaxy. The spin polarisation was studied by synchrotron-based photoelectron spectroscopy (PES). Magnetically dead layers of a few A thickness were observed and the behaviour is attributed to the superparamagnetic phase rather than the formation of nonferromagnetic compounds. For Fe on the Ga-terminated substrate, the spin polarisation increased abruptly and then remained at a constant value when the film was deposited in thickness. In contrast, Fe grown on the As-terminated GaAs(1 0 0) substrate reveals a variable behaviour of spin polarisation with two spin polarisation maximal, along with increasing the thickness of Fe coverlayers. The underlying decreases in the spin polarisation may be attributed to structural changes of Fe coverlayers, related to the different ternary metastable phases. (C) 2002 Elsevier Science B.V. All rights reserved.