[en] We have prepared magnetic beads consisting of iron oxide nanoparticles in a polymethyl methacrylate (PMMA) and a polyvinyl butyral (PVB) matrix. High-field Moessbauer studies show that the particles have an almost perfect collinear spin structure and magnetization measurements show that they are superparamagnetic at room temperature at a time scale of seconds. We have followed the oxidation of the particles, which initially have a stoichiometry close to magnetite. The oxidation is fast during the first 2-3 weeks and then continues slowly such that even after 30 weeks the particles have not completely transformed to maghemite. The PVB beads are hydrophilic and biocompatible and are therefore well suited for applications in medicine and biology