FexSey films were prepared on GaAs(001) substrates by a selenization of Fe films using molecular beam epitaxy equipment. Structural and magnetic properties of FexSey thin films during their selenization process were studied. The selenized films obtained consisted of polycrystalline grains of 100–700 nm. A magnetic anisotropy of in-plane/perpendicular to the films was weakened by increasing the selenization ratio of the samples, which was interesting in contrast to the fact that the grain size of the films became larger. © 1998 American Institute of Physics.