An experimental study was made to determine the effective thermal conductivities of magnesium oxide, stainless steel, and uranium oxide powders in various gases. The gases included air, helium, argon, nitrogen, neon, and various mixtures of these gases. The variables investigated included temperature level, gas pressure, porosity, gas conductivity, and solid conductivity. A reasonably good correlation of the data was obtained by using the dimensionless groups effective thermal conductivity over gas conductivity, solid conductivity over gas conductivity, and porosity of the powder, although the effective conductivity may also be a function of the shape and arrangement of the particles. A solution for spheres in cubical array was obtained by relaxation and was in reasonable agreement with the data for porosities in the neighborhood of that for the spheres.