The structural changes occurred in bioactive glass microspheres belonging to the system SiO2-Na2O-P2O5-CaO-K2O-MgO incorporating yttrium were investigated before and after soaking in simulated body fluid (SBF) by X-ray diffraction (XRD) and P-31 and Si-29 magic angle sample spinning nuclear magnetic resonance (MAS-NMR). The addition of yttrium to the bioactive glass composition induces changes in the behavior of the glass microspheres in SBF. The XRD analysis proves that after the immersion in SBF a crystalline hydroxyapatite-like phase is developed on the microspheres surface. The Si-29 and P-31 MAS-NMR results show that silicate species with two and three bridging oxygens per SiO4 tetrahedra and PO4 monomeric units are present in the glass structure. After immersion in SBF, new silicate species with four bridging oxygens appear as result of silica-gel layer formed on microspheres surface. The formation of crystalline hydroxyapatite-type layer is reflected by the occurrence of narrow components in P-31 MAS-NMR spectra. The NMR results support the Hench model for bioactive glasses behavior in biological environments.