The molybdenum-catalyzed coprocessing reactions of Illinois No. 6 or Wyodak coal with Lloydminster or Hondo resid have been investigated by using dideuterium under severe conditions : 3000 psi at 420-degrees-C, for 2 h. Asphaltene and coal conversions to lower molecular weight, less polar compounds exceed 70 and 90%, respectively, under these conditions. There are some differences in the product distributions for the four coal and resid mixtures, but these variations are rather modest. The product from Lloydminster resid and Illinois No. 6 coal yielded 10.2% gas, 75.1% oil, 3.9% resin, 8.5% asphaltene, and 2.3% insoluble material. The distribution of deuterium and hydrogen was measured in the gases, oils, resins, and asphaltenes. The deuterium contents of these products are large, near the statistical value dictated by the hydrogen/deuterium ratio in the starting materials. Evidence for modest selectivity is provided by the higher deuterium content at aromatic and reactive benzylic positions and the lower concentrations of this isotope at less reactive paraffinic sites. The purely paraffinic constituents in the oils, for example, experience only partial exchange, but the hydroaromatic molecules in the oils have abundant deuterium reflecting their origins in catalytic reduction reactions. The deuterium in the gaseous products suggests different origins for methane, ethane, and propane and butane. The results of this study and other recent work strongly infer that the success of the molybdenum-catalyzed reaction system depends upon the reduction of aromatic molecules, the facile deoxygenation of phenolic compounds, the removal of other heteroatoms via reductive chemistry, the fragmentation of large asphaltenes, especially the heteroatom-containing compounds, and the alkylation of aromatic molecules. These beneficial reactions occur simultaneously with other catalytic reactions that involve virtually all the molecules in the system. It is evident that carbon-hydrogen bond cleavage proceeds rather readily on the catalyst. Fortunately, the catalytic addition of hydrogen to the intermediates on the catalyst surface is a favored process at high pressure, and fragmentation and coke formation are thereby minimized.