Powdered samples (170-230 mesh) of a glass of composition 1.25CaO.SiO2 were soaked in a simulated body fluid (SBF). The powders were submitted to Fourier transform infrared transmission spectroscopy as coarse powders (such as drawn out from the SBF) and as fine powders (soaked and subsequently ground). Soaked samples were submitted to differential thermal analysis (DTA) and the crystalline phases formed during heating in the DTA apparatus were identified by means of X-ray diffraction analysis. The method appears to be useful in studying the mechanism of deposition of the hydroxyapatite layer. It is documented, by using the same method, that the mechanism involves the reactions of hydrolysis and successive condensation and repolymerization of the silicate substrate. These reactions are very fast. Extensive Ca2+ cation depletion occurs, but appears to be slower.