A study of the structure and bonding configuration of the bioactive glasses in the system Na2O-CaO-P2O5-SiO2 by Fourier transform Raman spectroscopy is presented. The assignment of the Raman lines, the changes in the Si-O-Si bond environment and the identification of the non-bridging silicon-oxygen groups (Si-O-NBO) for a wide range of silicate glasses are discussed. The frequency shifting and intensity variations of the Raman lines as a function of the bioactive glass composition are attributed to a decrease of the local symmetry originated by the addition of alkali and alkali earth oxides to the vitreous silica network. Correlation plots for the quantification of the Si-O-NBO groups as a function of the glass composition are also presented. These Raman analyses contribute to a better knowledge of the structural role of the network modifiers in the bioactive glasses and, as a consequence, improve the understanding of the bioactive process and the chemical routes of the CaP layer formation when exposed body fluids. © 2003 Elsevier Science B.V. All rights reserved.