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Eu-doped silica nanoparticles with different concentrations of europium (3 %, 6%and 15 at. %) were prepared by a hydrothermal procedure. Light scattering measurements of the silica colloids as well as TEM microscopy, revealed a narrow size distribution of the particles, ranging from 5 to 10 nm. X-Ray diffraction showed that all the powders were amorphous. Magnetic susceptibility vs. temperature measurements showed a behaviour typical for Eu3+ ions, with a plateau below 60 K, and an increase of the mass susceptibility with Eu concentration. The splitting of zero-field cooled (ZFC) and field-coled (FC) susceptibility branches increased with the europium concentration, indicating the presence of interparticle interactions. At temperatures below 40 K, a contribution of paramagnetic impurities obeying the Curie law was observed. Heat treatment of the powders at 823 K and 1073 K increased the magnetic susceptibility. The appearance of a small peak at 70 K indicated the presence of small quantities of ferromagnetic EuO in the powders after heating.