We have studied the optical and structural properties of Eu2O3 thin films grown by RF magnetron sputtering on Si substrates. The films have been annealed in O2 ambient to improve their properties. The intensity of the photoluminescence (PL) signal detected at room temperature from the films depends on the temperature of the thermal process. The structural characterization of the films, performed by transmission electron microscopy, energy filtered transmission electron microscopy and x-ray diffraction, reveals that annealing processes performed at temperatures higher than 900 °C induce a mixing at the Eu2O3–Si interface, leading to the formation of a silicate-like layer, which is responsible for the observed decrease of the PL intensity.