A study of the short-range organization of bioactive SiO 2 –P 2 O 5 –CaO sol–gel glasses using high-resolution transmission electron microscopy, Fourier transform infrared and magic angle spinning—nuclear magnetic resonance spectroscopies are reported. These combined techniques were used to investigate the rearrangement of the glass matrix and to clarify the role of phosphorus and calcium during the heat-treatment stage of the sol–gel process. The study shows how the incorporation of calcium into the glass structure promotes the appearence of P–O–NBO groups and that the glass nanostructure is induced to form Ca-rich orthophosphate units.