Three glasses in the system CaO-MgO-SiO ₂ -P ₂ O ₅ , with high silica content (80 mol% SiO ₂ ) and 0, 3 and 7 mol% of MgO, respectively, have been prepared by the sol-gel method. The in vitro bioactivity of pellets, prepared by compacting the glass powders, was assessed by determining the changes in surface morphology and composition after soaking in a simulated body fluid (SBF) for up to 14 days at 37 °C. Formation of a calcium phosphate rich layer on the glasses surface was followed by scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Three effects of MgO in the glass upon the newly formed layer were detected: (i) it slows down the rate of formation of the layer, (ii) it increases the thickness of the layer and (iii) a whitlockite-like phase appeared together with the apatite-like phase formed on the surface of bioactive materials when exposed to physiological solutions.