The conversion of a bioactive borosilicate glass in aqueous phosphate solution was observed to produce vastly different reaction kinetics and hydroxyapatite (HA) microstructures, depending on whether the glass was reacted continuously or intermittently in the solution. Particles (150-300 mu m) of a borosilicate glass (designated H12) were reacted continuously or intermittently in 0.25M K2HPO4 solution with a starting pH value of 7.0 at 37 degrees C. The conversion kinetics of the glass particles to HA were determined from weight loss measurements. Structural and compositional changes resulting from the conversion reaction were characterized using scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray analysis, and Fourier transform infrared spectroscopy. For conversion experiments carried out intermittently (12-24 h intervals, followed by drying), faster reaction kinetics and a unique multilayered microstructure, consisting of alternating layers of HA and an amorphous SiO2-rich material with nearly uniform thickness (2-3 mu m), were observed. On the other hand, particles reacted continuously in the phosphate solution for the same total time converted more slowly and produced a single HA layer. The kinetics and mechanism of forming HA under the intermittent and continuous reaction conditions are described and compared with those for bioactive silicate and borate glasses studied in previous work.