"Values of absolute permeability to nitrogen and carbon dioxide are presented for Pricetown bituminous, Hanna and Gillette sub-bituminous and Texas lignite coals. These values as well as those for porosity and shrinkage are found to be inversely proportional to rank . Effective and relative permeability's to gas as functions of water saturation, overburden pressure and gas type are measured under pseudo steady-state low Reynolds Number flow. The effect of carbon dioxide is modest as regards decreases in permeability. The effect of overburden pressure is quite pronounced for the effective permeability's to gas. Relative permeability's are insensitive to both gas-type and overburden pressure but are quite distinct with regard to coal type. Critical gas saturation values cover a wide range for the various coals but do not correlate with rank."