Fe and Fe40Co60 thin films (5-60 nm) with a bcc structure have been prepared on a-axis oriented sapphire substrates by molecular beam epitaxy. The structural properties have been characterized in situ by reflection high-energy electron diffraction and ex situ by X-ray diffraction and atomic force microscopy. A two-dimensional magneto-optical Kerr effect set-up has been used to determine the in-plane magnetization components and to investigate the magnetic anisotropy and the orientation dependence of the magnetization reversal process. The Fe films and Fe40Co60 alloy films both display a uniaxial in-plane anisotropy. They also exhibit a comparable increase of the coercive field along the easy axis with increasing thickness. We have evaluated this dependence using the results of the structural characterization, indicating that the enhancement of the coercive field is linked to the growing surface roughness and decreasing structural coherence. (C) 2002 Elsevier Science B.V. All rights reserved.