Eight coals, ranging in rank from subbituminous B to high-volatile A bituminous, were reacted in microautoclave reactors in a hydrogen atmosphere and with the nondonor solvent phenanthrene. Reactions were conducted at 360 degrees C for 1 h. Both noncatalytic and catalytic reactions were investigated. The catalytic reactions employed ammonium tetrathiomolybdate as a catalyst precursor. In noncatalytic reactions, conversions ranged from 18 to 47% and showed a dependence on the oxygen contents of the coals. Addition of a catalyst increases conversions to 54-83%. In catalytic reactions there is no apparent dependence of conversion on oxygen content, but rather the hydrogen content, the net hydrogen (the total hydrogen corrected for oxygen, sulfur, and nitrogen), and its distribution in the coal structure are important. Segregation of the coals into groups originally defined by Given shows two different effects of added catalyst : far medium- to low-rank medium-sulfur coals the available hydrogen in the coal and that supplied by the catalyst act additively, while for high-rank medium-sulfur coals there is a synergistic effect of added catalyst. At these reaction conditions, use of the catalyst does not shift the product slate markedly; rather, the catalyst increases conversion by enhancing the yields of individual products in the same relative proportions as were formed in noncatalytic reactions.