In this paper, processing and properties of water durable solid electrolyte membranes prepared from a glass-ceramic powder in the Li2O-Al2O3-TiO2-P2O5 system are reported. Specifically, ionic conductivity, microstructure, and water stability of 150 to 350 mu m thick membranes of the glass-ceramic and its composite with polyethylene are investigated and discussed. The grains (crystallites) of the glass-ceramic show liquidlike conductivity at ambient temperature, whereas the grain boundary conductivity is lower by a factor of about 3. The ratio between the grain and grain boundary conductivities in the temperature range of 213 to 300 K varies by a factor of 3 to 14. The lower the temperature, the larger the ratio. The difference in conductivities is explained on the basis of crystallographic mismatch and the existence of crystalline AlPO4 at the grain boundary. The membranes were very stable in tap water. The polyethylene infiltration further improved water stability with little impact on conductivity.