The effective particle size of unconsolidated materials on the Martian surface can be determined from thermal inertia, due to a pore size dependence of thermal conductivity at Martian atmospheric pressures. Because dunes consist of a narrow range of well-sorted, unconsolidated particles, they provide for a test of the relationship between particle size and thermal inertia calculated from midinfrared emission data for the Martian surface. We use two independent approaches. First, thermal inertia data indicate that Martian dunes have an average particle size of about 500¿100 μm, or medium to coarse sand. Second, we determine expected dune particle sizes from grain trajectory calculations and the particle size transition from suspension to saltation. On Earth, the transition occurs for a grain when the ratio of the terminal fall velocity to the wind friction speed (u*t) is near unity; for grains at u*t, this occurs at about 52 μm. Terrestrial dune sands have a mean of 250 μm and are composed entirely of grains>52 μm. The corresponding Martian transition grain size is about 210 μm, suggesting that Martian dunes should be significantly coarser than terrestrial dunes. Grain saltation path length as a function of particle size also shows that under Martin conditions, larger grains than on Earth will become suspended. Both approaches indicate that Martian dune sand should be coarser than terrestrial dune sand. Thus, while terrestrial dune grains are in the fine to medium sand range, the average Martian dune sediments are probably medium to coarse sands. These results closely match the grain sizes determined from thermal inertia models, providing the first direct test of the validity of these models for actual Martian surface materials. ¿American Geophysical Union 1991