Isothermal pyrolysis experiments were conducted at 375/degree/, 400/degree/, and 425/degree/C on Illinois No. 6 high volatile bituminous and on Wyodak subbituminous coal samples. Pyrolysis studies were conducted on nominally 10- to 20-gram samples using a fluidized sand-bath reactor system. A helium sweep gas flow rate of 30 cc/min was used to remove the products from the reaction zone. The amount of coal converted to tar, gas, and residue was determined for different reaction durations at each temperature. Elemental analyses (CHN and S) were obtained on the solid and liquid reaction products using standard instrumental techniques. Solid- and liquid-state /sup 13/C NMR techniques were used to characterize the residue coal and tars produced during varying stages of pyrolysis. The residue coal from each isothermal pyrolysis experiment was extracted with chloroform to obtain information about the chloroform soluble intermediates. Only small amounts of material were extractable with chloroform at any stage of pyrolysis for either coal. The distributed activation energy model was used to calculate activation energies for hydrocarbon gas evolution. Activation energies for individual hydrocarbon gas species were comparable for both coals, and were in the range of 48 to 56 kcal/mol. 35 refs., 18 figs., 4 tabs.