In this paper we analyze the kinetics of short-term physicochemical reactions at the interface between bioactive glass particles and biological fluids. Assessment of these reactions requires knowledge of elemental distribution under the micrometer scale at the interface between the bioactive glass and biological fluids. This analysis was performed by scanning transmission electron microscopy associated to energy-dispersive X-ray spectroscopy. But, microanalysis of diffusible ions such as oxygen, sodium, potassium, and magnesium is a major problem. To preserve the chemical identity of specimens, we used cryomethods for the preparation and the characterization of bioactive glass particles immersed in biological fluids. From 24 to 72 h, interactions between bioactive glass particles and biological fluids lead to a release of all elements and to the formation of three surface layers of the order of 300 nm in thickness. A first layer enriched in Si-Al, a second layer enriched in Ca-P-Mg (with a Ca/P approximate to 1), and a third layer enriched in Na-O. This paper demonstrates the formation of an exchange layer between Na+ ions from the glass and H3O+ from the solution. After 72 h, this layer disappears. The two other layers grow in thickness. The first layer becomes a pure Si layer with an increase of Si concentration. Ca, P, and Mg concentrations increase in the second layer, and the Ca/P ratio becomes near 1.7. The Si-rich layer permits the diffusion of Na, K, Ca, P, and Mg with a t(-1/2) law and the precipitation of an apatite layer at the materials periphery. The apatite layer grows in thickness with time and is of the order of 3 mum after 672 h of immersion in biological fluids.