Cristobalite is formed when borosilicate glass (Corning Code 7740) is sintered at temperatures ranging from 700-degrees to 1000-degrees-C. The precipitation kinetics, determined by XRD analysis, exhibit a characteristic incubation period which decreases with increasing sintering temperature, from 60-120 min at 700-degrees-C to 3-5 min at 1000-degrees-C. Activation analysis of precipitation shows an activation energy of 75 kJ/mol, which is close to that for the diffusion of Na+ in borosilicate glass, suggesting mass-transport-controlled kinetics.1,5 With added alumina content greater than a critical value, however, the cristobalite formation in the borosilicate glass is completely prevented at the sintering temperatures investigated. The critical alumina content is found to decrease with decreasing alumina particle size but with increasing sintering temperature. The above result, similar to observations previously made in a binary glass mixture containing a low-softening borosilicate glass (BSG) and a high-softening high silica glass (HSG)3 is attributed to a strong coupling between Al3+ from alumina and Na+ from borosilicate glass. The coupling reaction causes segregation of Na+ in borosilicate glass to alumina, thus forming a Na+- and Al3+-rich reaction layer around alumina particles far too rapid for cristobalite formation.