The effect of replacing varying amounts (0-2.5 mol.%) of B 2 O 3 with Al 2 O 3 in a borate glass on (1) the conversion of the glass to HA in an aqueous phosphate solution and (2) the compressive strength of the as-formed HA product was investigated. Samples of each glass (10 Â 10 Â 8 mm) were placed in 0.25 M K 2 HPO 4 solution at 60 °C, and the conversion kinetics to HA were determined from the weight loss of the glass and the pH of the solution. The structure and composition of the solid reaction products were characterized using X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. While the conversion rate of the glass to HA decreased considerably with increasing Al 2 O 3 content, the microstructure of the HA product became denser and the compressive strength of the HA product increased. The addition of SiO 2 to the Al 2 O 3 -containing borate glass reversed the deterioration of the conversion rate, and produced a further improvement in the strength of the HA product. The compressive strength of the HA formed from the borate glass with 2.5 mol.% Al 2 O 3 and 5 mol.% SiO 2 was 11.1 ± 0.2 MPa, which is equal to the highest strengths reported for trabecular bone. The results indicated that simultaneous additions of Al 2 O 3 and SiO 2 could be used to control the bioactivity of the borate glass and to enhance the mechanical strength of the HA product. Furthermore, the HA product formed from the glass containing both SiO 2 and Al 2 O 3 could be applied to bone repair.