Bioactive glasses possess the ability to bond to living tissues through the formation of a calcium phosphate-rich layer at their interface with living tissues. This paper reports the different steps of this bioactivity process via a complete micro-PIXE characterization of a sol-gel derived SiO2-CaO bioactive glass in contact with biological fluids for different delays. Multi-elemental cartography at the glass/biological fluids interface together with major and trace elements quantification permit a better understanding of the five reaction stages involved in the bioactivity mechanisms. The presence of phosphorus was detected at the periphery of the material within 6 h of interaction with biological fluids. A calcium phosphate-rich layer containing magnesium is formed after a few days of interaction and presence of bone-like apatite is deduced from the calculation of the Ca/P ratio at the material interface. That is of deep interest for clinical applications, because this biologically active behavior results in the formation of a strong interfacial bond between the glass and host tissues, and will stimulate bone-cell proliferation.