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Helium permeation, diffusion, and solubility have been measured for a number of lithium silicate and aluminosilicate glasses. Helium mobility in the lithium silicate glasses is controlled by the two‐phase morphology of these glasses. Addition of as little as 2 mol% alumina strongly suppresses the tendency toward phase separation, i.e., yields homogeneous glasses. The compositional dependence of helium mobility in these glasses indicates that a major change in the glass structure occurs when the alumina‐to‐lithia ratio exceeds unity. Comparison of these results to those for analogous sodium aluminosilicates indicates that lithium has a greater effect than sodium on the free volume of the glass.