Abstract Preliminary studies were made on the organic matter associated with fossil logs and uranium-rich ores of the Morrison Formation in the San Juan basin, New Mexico, using solid-state 13 C nuclear magnetic resonance, stable carbon isotope ratios, and elemental analyses. These studies show that information about the structure of the organic matter can be obtained that could lead to a better understanding of its origin and nature. The nuclear magnetic resonance data indicate that the apparently non- cellular organic matter in these ores is highly aromatic, and confirm elemental analyses that show low atomic H/C ratios and high O/C ratios. Information from the nuclear magnetic resonance spectra, as well as the elemental data, led us to believe that this organic matter is more like a medium-rank coal than like the “amorphous carbon” described by previous workers. Trends in the nuclear magnetic resonance, elemental, and carbon isotope data are consistent with the hypothesis that the organic matter originated as humic acids. The trends also suggest that factors other than radiation damage could be responsible for the chemical structural nature and partially responsible for the stable carbon isotope composition of the organic matter. These factors include the early oxidation of plant material to form humic acids, which imparts structural characteristics different from those of the parent material, and the possibility that different types of plant material are involved in the formation of ores.