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Leach tests on borosilicate glasses using a partial-replenishment technique show that in certain cases large excursions in leach rate may appear at the end of periods ranging between 1 and 20 months. The extent and time of these leach rate transients are very sensitive to glass and leachant composition and to S/V ratio, and they may be eliminated by slight increases in Si and Al levels in the glass or by using a neutrally buffered leachant. This phenomenon, which may reflect cracking of the initial surface layer, can have a considerable effect on the results and complicate the modeling of leach tests which strongly reflect the extent of leaching during the early stages of glass/leachant interaction. However, these effects appear to have relatively little significance in the long term.