This paper reports, for the first time, observation of sassolite (crystalline B(OH)(3)) formation in alkali borosilicate glass (BSG) powder due to hydration during aging. Correlation between hydration behavior, sassolite formation, and phase separation has been established and hydration-resistant low-alkali BSG powders were successfully prepared. The glass composition exhibited very low moisture absorption of < 3 wt% in powder form, for an aging duration of 6000 h. This has been achieved by carefully tailoring the glass transition temperature (T(g)) and critical immiscibility temperature (T(c)). We also report a new, simple, and effective sample preparation method for detecting phase separation in BSG, using transmission electron microscopy.