Biocompatible composites, consisting of magnetite nanoparticles embedded into a matrix of meltable dextran ester are presented, which can be softened under an induced alternating magnetic field and may thereby allow magnetically controlled release applications. Temperature dependent mobility investigations of magnetic nanoparticles in the molten composites were carried out by optical microscopy, AC susceptibility and Mössbauer spectroscopy. Optical microscopy shows a movement of individual agglomerates and texturing in the micrometer scale, whereas AC-susceptometry and Mössbauer spectroscopy investigations reveal that the particles perform diffusive Brownian motion in the liquid polymer melt as separated particles rather than as large agglomerates.