Bioactive glass powders, with a composition of Si02-CaO-P205, have been successfully synthesized via a sol-gel process at considerably lower temperatures than required for conventional melting methods. Bioactive glass powders made via conventional methods form a n interfacial bond with hone when they are implanted. Bonding is correlated with the formation of a surface hydroxyapatite layer. This study examined the formation of a hydroxyapatite layer in Tris-buffered solution as a function of SiO, content of sol-gel derived powders. A FT-IRRS technique was used to monitor the formation of the hydroxyapatite on the surface of the powders. X-ray diffraction analysis and BET were also used to characterize the chemical and physical properties of the sol-gel derived bioactive powders. It was discovered that: (a) the rate of hydroxyapatite formation decreased with increasing S i 0 2 content for powders whose SiOz content was less than 90 mot%; (b) a hydroxyapatite film does not form for the powders whose SiOt content is more than 90 mol%; (c) the SiO, limit, beyond which the powders lost their hioactivity, was much higher for bioactive glass powders made through sol-gel process (90%) than those made by conventional melting methods (60%). These results indicate that it is possible to significantly expand the bioactive composition range through microstructural control made possible by sol-gel processing techniques.