Geological models for petroleum generation suggest thermal conversion of oil-prone sedimentary organic matter in the presence of water promotes increased liquid saturate yield, whereas absence of water causes formation of an aromatic, cross-linked solid bitumen residue. To test the influence of exchangeable hydrogen from water, organic-rich (22 wt.% total organic carbon, TOC) mudrock samples from the Eocene lacustrine Green River Mahogany zone oil shale were pyrolyzed under hydrous and anhydrous conditions at temperatures between 300 and 370°C for 72 hrs. Petrographic approaches including optical microscopy, reflectance, Raman spectroscopy, and scanning electron and transmission electron microscopy, supplemented by geochemical screening measurements (TOC content and programmed pyrolysis), were used to quantify differences in relative appearance, abundance and composition of solid bitumen newly generated during the pyrolysis experiments. Results show hydrous residues contain lower TOC, comprised of solid bitumen with higher aromaticity, and textures indicative of lower viscosities, than anhydrous residues from the same temperature pyrolysis conditions. These observations suggest solid bitumen forming from thermal conversion of oil-prone sedimentary organic matter under anhydrous conditions is less aromatic, although more cross-linked, than solid bitumen forming under hydrous conditions at the same time-temperature combination. A radical disproportionation mechanism favored in the presence of hydrogen radical donation from water promotes aromatization in the solid residue with concomitant expulsion of saturated hydrocarbons.