X-ray photoelectron spectroscopy has been used to study the (0001) cleavage face and a perpendicular face of a Na β-alumina single crystal exposed to ambient atmosphere. Quantitative analysis, which took into account the particular anisotropic structure of this solid and its expected effect on XPS relative line intensities, revealed Na-O rich ovelayers at the latter face. Detailed in-depth composition profiles showed that this Al-free region is several monolayers thick and that some Na-O enrichment extends deeper into the solid. The phenomena are attributed to interactions of the mobile Na+ ions with H2O molecules from the air causing Na+ to segregate at this surface. The anisotropy of the effect is closely related to the two-dimensional fast ion transport in Na β-alumina, and it has implications for the performance of Na/S electrochemical cells which use this solid as the electrolyte.