A stable Eu^3+?Eu^2+ reduction is accomplished by thermal annealing in N_2 ambient of Eu_2O_3 films deposited by magnetron sputtering on Si substrates. Transmission electron microscopy and x-ray diffraction measurements demonstrate the occurrence of a complex reactivity at the Eu_2O_3/Si interface, leading to the formation of Eu^2+ silicates, characterized by a very strong (the measured external quantum efficiency is about 10%) and broad room temperature photoluminescence (PL) peak centered at 590 nm. This signal is much more efficient than the Eu^3+ emission, mainly consisting of a sharp PL peak at 622 nm, observed in O_2-annealed films, where the presence of a SiO_2 layer at the Eu_2O_3/Si interface prevents Eu^2+ formation.