The effect of magnetite nanoparticles (NPs) content on the magnetic properties of polylactide (PLA) and polystyrene (PS) matrix has been investigated by means of DC magnetization measurements as a function of temperature (T) and magnetic field (H). Previous to the dispersion by melt-compounding into PLA and PS, the magnetite NPs have been reactively surface treated with 3% polymethylhydrogensiloxane (MHX) in order to make them hydrophobic and more stable to the action of oxygen and moisture. The magnetic analysis of the properties has been performed by measuring the Zero Field Cooling (ZFC) magnetization curve as a function of the temperature, at 0.1 Tesla applied field. In this framework, a superparamagnetic shape like has been noted for all the samples with the possibility to individuate the blocking temperature (TB) of the NPs. Moreover, the magnetization as a function of the field has been measured at room temperature (in particular above TB) investigating the coercive field and the magnetization values finding potentially interesting results. In particular, the very low values obtained for the coercive field at room temperature, together with the maximum found in the ZFC curve, have confirmed the superparamagnetic behavior of the PLA and PS - magnetite filled samples. Finally, a table with the fundamental magnetic features values of the samples has been reported for understanding if the obtained results for this kind of surface treated NPs and the afferent nanocomposites could be suitable for being used in applications requiring superparamagnetic properties (protection of environment, magnetic microcarriers, magnetic separation of stem cells, other biomedical purposes).