One persistent difficulty associated with the interpretation of Martian thermal inertia and the derivation of particle size from it has been the degree to which atmospheric properties affect both the energy balance at the surface and the gas conductivity. Because both these factors vary with atmospheric pressure, derivation of thermal inertias and particle sizes must incorporate a correction for elevation. Here, by utilizing currently available thermal models and laboratory data, fine component thermal inertia has been combined with topography to compute particle size corrected for these elevation-dependent effects. The correct particle sizes at high elevations are as much as 2 orders of magnitude larger than the uncorrected values, with the Tharsis shield volcanoes generally having inferred sizes larger than dust, in contrast to previous interpretations of a thick dust cover. This large effective particle size may be a manifestation of the average conductivity produced by a mixture of dust and intervening bedrock outcrops smaller in size than those computed by Christensen [1986b>. In addition, the time-variable albedo in the upper regions of these volcanoes argues against the presence of large dust deposits. ¿American Geophysical Union 1994