The presence of sulfur dioxide (SO2) on Io, together with the fact that the surface layer of Io has extremely high porosity, suggests the possibility of diffusion of this volatile within the surface, as well as exchange between the surface and an atmosphere. We investigate the former possibility through the development of a surface layer thermal model and subsequent calculations of the thermally driven diffusion flux of SO2 within the layer. The major factors affecting the diffusion process are the temperature and temperature gradient in the surface layer throughout the day (which are results of the thermal model), and the porosity and grain size in the surface layer. Our results indicate that the net transport of SO2 in the near-surface region is downward into the subsurface, causing near-surface depletion of SO2. Near-surface depletion would result in a layer of reduced thermal inertia overlying the bulk of the surface, consistent with thermal eclipse observations of Io. For our nominal model with 10-μm grains and a porosity of 85%, the peak net diurnal downward flux reaches nearly 8¿10-3 g cm-2 period-1. ¿American Geophysical Union 1991