In this work we report an irreversible behavior in the magnetization of oleic acid (OA)-coated magnetite nanoparticles (NPs) at high temperature. On one hand, when the sample is heated-up, an irreversible magnetization drop is observed at 550 K, while the Curie temperature (Tc ≈ 840 K) remains close to that of bulk magnetite. On the other hand, a significant reduction of the magnetic moment has been found after heating and cooling the sample above and below its Curie temperature, respectively. Both magnetic features have been also observed in other surface modified magnetite nanoparticles [e.g., polyvynil pyrrolidone (PVP)-coated]. However, no reduction of the magnetic moment upon a heating-cooling cycle has been found in ligand-free magnetite nanoparticles. This fact suggests that surface effects derived from the OA-coating could be the origin of such reduction of the magnetic moment, as a consequence of thermal decomposition of the organic layer surrounding the nanoparticle and the subsequent loss of the magnetic surface order. This feature could be an important factor for those applications requiring high temperatures.