The process of apatite formation on the surface of Na2O-SiO2 glass in a body environment was investigated, mainly by X-ray photoelectron spectroscopy, as a function of soaking time in a simulated body fluid (SBF). The glass was found to release Na+ ions via exchange with H3O+ ions in the SBF to form Si-OH groups on its surface. These Si-OH groups induced apatite formation indirectly, by forming calcium silicate and amorphous calcium phosphate. The formation of the calcium silicate and amorphous calcium phosphate is attributed to electrostatic interactions between the Si-OH groups on the glass surface and the calcium and phosphate ions in the SBF.