Both methane and helium flow through solid coal by Knudsen diffusion, even at relatively high pressures. Knudsen permeabilities were determined for samples of Pittsburgh, Pocahontas No. 3, and Oklahoma Hartshorne coals. The average permeability for dry methane was 1.3 X 10-6 cm2/sec for Pittsburgh coal, 20 x 10-6 for Pocahontas No. 3 Coal, and 13.8 X 10-6 for for Oklahoma Hartshorne coal. A molecular-sieve effect exists for methane in all three coals examined and is very strong in Pittsburgh coal. If the methane is saturated with water vapor, the permeability decreases by a factor of 3 to 25, depending on the coal. The knudsen permeability of solid coal disks seems to bear no relation to coalbed permeability. Lag-time measurements indicate systems of both larger and finer pores and the presence of blind pores. This effect is again more pronounced in the Pittsburgh coal.