Short-term physico-chemical reactions at the interface between bioactive glass particles and biological fluids are studied for three glasses with different bioactive properties; these glasses are in the SiO(2)-Na(2)O-CaO-P(2)O(5)-K(2)O-Al(2)O(3)-MgO system. Our aim is to show the difference between the mechanisms of their surface reactions. The relation between the composition and the bioactive properties of these glasses is also discussed. The elemental analysis is performed at the submicrometer scale by scanning transmission electron microscopy associated with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. After different immersion times (ranging from 0 to 96 h) of bioactive glass particles in a simulated biological solution, results show the formation of different surface layers at the glass periphery in the case of two bioactive glasses (A9 and BVA). For the third glass (BVH) we do not observe any surface layer formation or any modification of the glass composition. For the two other glasses (A9 and BVA), we observe the presence of different layers: an already observed (Si, O, Al) rich layer at the periphery, a previously demonstrated thin (Si, O) layer formed on top of the (Si, O, Al) layer and a (Ca, P) layer. We determine the different steps of the mechanisms of the surface reactions, which appear to be similar in these glasses, and compare the physico-chemical reactions and kinetics using the different immersion times. The A9 glass permits the observation of all important steps of the surface reactions which lead to bioactivity. This study shows the important relationship between composition and bioactivity which can determine the medical applicability of the glass.