Preliminary measurements of the effects of bulk density and particle size sorting on the thermal conductivity of particulate materials under Martian atmospheric pressures are presented and discussed. Concoidally fractured particles tend to form more loosely packed, less dense sedimentary structures, due to irregularities in the shape of the particles, than those formed by spherical particles of similar size. The lower bulk density of the angular-shaped particles leads to a lower thermal conductivity of the sample. If the density difference is assumed to be the sole factor that controls the difference in conductivity in this case, then the thermal conductivity of 25--30 μm size particles appears to increase linearly with increasing bulk density and with the square root of the atmospheric pressure. Initial experiments appear to indicate that the bulk thermal conductivity of a particulate material containing a mixture of different particle sizes is the same as the thermal conductivity that a material of similar bulk, density would have if it were composed entirely of the largest particle size contained within that material. More studies are, however, necessary to confirm this apparent trend. ¿ 1997 American Geophysical Union