Ab initio and model calculations demonstrate that the spin motion of electrons transmitted above the vacuum energy through ferromagnetic films can be investigated by means of angle- and spin-resolved core-level photoelectron spectroscopy. The motion of the photoelectron spin polarization can be regarded as a combination of a precession around and a relaxation towards the magnetization direction. For ultrathin Fe films on Pd(001), its dependence on the Fe film thickness and on the Fe electronic structure is studied systematically. In addition to elastic and inelastic scattering, the effect of band gaps on the spin motion is addressed in particular.