Helium permeation, diffusion, and solubility have been measured for lithium, sodium, potassium, rubidium, and cesium borate glasses. The compositional dependence of each of these properties is related to the compositional dependence of the molar volume. Initial additions of alkali oxides (with the possible exception of lithium) to vitreous B2O3serve to compact the glass network and reduce helium mobility and solubility. However, at higher alkali oxide concentrations, the network is expanded and helium diffusion actually increases with increasing alkali oxide concentration. These results are contrary to any previously reported for glasses, as inert gas mobility has always been reported to decrease with increasing alkali oxide content. However, the molar volume considerations used here can be applied to previous studies with equal success.