Two techniques, infrared (IR) and X-ray induced photoelectron spectroscopies (XPS) were used to study a wide range of glasses in,the system Na2O-CaO-P2O5-K2O-MgO-B2O3-SiO2 that have been shown elsewhere to be bioactive. The IR frequency shifting of the bridging oxygen stretching vibration (Si-O-Si) and the binding energy shifting of the O1s and Si2p photoelectron peaks have been observed and evaluated. Both effects are originated by the incorporation of the alkali ions into the vitreous silica network, which provokes a modification of the local symmetry and of the electronic density around the different atoms. The presence of non-bridging silicon-oxygen (Si-O-NBO) groups in the glass have been identified, and important correlations between the concentration of Si-O-NBO groups and the content of network modifiers were found. This information is very useful for the development and tailoring of new bioactive glasses with an optimum bioactivity. This study leads to a better knowledge of the structure and bonding configuration of the bioactive glasses and, consequently, can contribute to clarify the role of the network modifiers in the bioactive process. (C) 2003 Elsevier B.V. All rights reserved.