The Na atmosphere of Io requires a Na-S-O phase in the outer surface layers. Considering the various mechanisms for extraction of Na to the surface, the possible primary phases are Na2O, Na2Sx, and Na2SO4. However, regardless of the primary phases brought to the surface, the shallow crustal recycling of material implied by the ongoing volcanism will tend to produce thermochemical equilibrium and cause all Na to end up as Na2SO4 or Na2Sx. This hypothesis is investigated by relatively model-independent thermodynamic calculations. The major assumption is that material is statistically circulated to sufficiently high temperatures by burial that thermochemical equilibrium can be attained. For a wide range of assumed crustal (PT) conditions, Na2O will be converted to Na2SO4. During residence in the shallowest crustal regions dominated by liquid SO2, e.g., SO2 geysers or fumaroles, or for any crustal regimes where SO2 and S are in comparable abundances, Na-sulfides will be converted to Na2SO4. However, in high-temperature, low-pressure regimes with a low relative abundance of SO2 relative to S (e.g., due to outgasing of SO2), Na2SO4 is converted to Na sulfides. Such regimes could be relatively common on Io, e.g., associated with flows, lava lakes, or shallow intrusions. Consequently, because of thermochemical equilibration in different crustal environments, both Na2SO4 and Na sulfides will coexist on the Io surface. ¿ American Geophysical Union 1995.