Sodium metal can be made by electrolysis of molten sodium hydroxide in sodium beta '-alumina membrane electrolysis cells. However, some uncertainty is about the lifetime of the sodium beta '-alumina membranes in contact with molten sodium hydroxide. The main objective of the work described here is to address this uncertainty and to study the corrosion of the membranes in molten sodium hydroxide. Samples of sodium beta '-alumina disk were aged in molten sodium hydroxide for up to 149 days. Mass loss was only 0.15% in the first 108 days, but in the next 41 days mass loss accelerated considerably, or up to 0.33% in 41 days. No change in phase composition of the material was observed in the 149 days. Investigation of aged samples with scanning electron microscope revealed that the molten sodium hydroxide preferably dissolved material at grain boundaries, which most likely resulted in freeing of grains (1-5 mu m diameter) from the bulk of the material.