Rare earth-containing bioactive glasses are promising materials for biomedical applications like brachytherapy, luminescence-based imaging, magnetic resonance imaging, among others, due to the electronic configuration of rare earth. Although studies of such glasses have continuously been increasing, they still have been poorly studied as biomaterials. In this work, we studied the influence of gadolinium and ytterbium on the structure of bioactive glasses and iron-based bioactive glasses. The glasses were obtained by melt quenching, and their structures were characterized by X-ray diffraction, nuclear magnetic resonance, X-ray photoelectron spectroscopy, electron paramagnetic resonance and scanning electron microscopy. The results showed that the addition of rare earth elements in the glass structure creates more non-bridging oxygen bonds in the silicate network, to satisfy the condition of octahedral coordination of the rare earth ions. An observed Q(3)Q(2)+Q(1) depolymerization of the glass network is a factor that could favor bioactivity. In conclusion, our experimental data address the bioactive behavior of rare earth-containing glasses to an increased concentration of non-bridging oxygen bonds and a more fragmented network.