The ability of bioactive glass to function as an effective bone graft material is directly related to its in vivo dissolution, ion release kinetics, and formation of bone-like hydroxyapatite layer. A spherical shape represents an optimal geometry for controlling and predicting the ion dissolution kinetics of bioactive glasses. In this work, microspheres of S53P4 (mol.%: 53.9 SiO2, 22.7 Na2O, 21.8 CaO, 1.7 P2O5) were manufactured via flame-spraying [1]. Spherical and irregular crushed particles of S53P4 with a size fraction of 45-90 µm were used to compare the ion release and the formation of reaction layers in a continuous flow of TRIS-buffer solution. The particle size distribution and surface morphology were evaluated using scanning electron microscopy (SEM) and laser diffraction analysis. The dissolution of the glass granules and microspheres was performed at 37 ℃ using the flow rate of 0.2 ml/min. The ion release was analyzed using inductively coupled plasma atomic emission spectroscopy, and pH changes were measured along the dissolution. The presence of eventual SiO2-rich and calcium phosphate (CaP) surface layers after 4, 8, and 24 h of dissolution was analyzed using SEM-EDX. After 24 h, the mass loss was greater from the granules (50%) than from microspheres (40%). Thus, the particles with sharp edges and aspect ratios provided a faster release of silicon, calcium, sodium species, and subsequently higher pH changes due to a larger surface area. The release of phosphorous was higher from the surface of microspheres. SiO2-rich layers were identified after 4 h of dissolution of granules and microspheres, while CaP was detected locally on the microspheres (Figure 1). The layer thicknesses increased with dissolution time for both granules and spheres, i.e. SiO2-rich layer up to 20 µm for granules and up to 15 µm for spheres after 24 h of dissolution. Interestingly, precipitated CaP layer was thicker for microspheres (up to 3 µm) than for granules (up to 1.5 µm). It appears that the use of bioactive glass microspheres with their uniform shape helps to better control the reaction layers and provides a good platform for modeling of dissolution kinetics under dynamic conditions.